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Strengthening Forensic Science in the United States: A Path Forward 5 Descriptions of Some Forensic Science Disciplines This chapter describes the methods of some of the major forensic science disciplines. It focuses on those that are used most commonly for investigations and trials as well as on those that have been cause for concern in court or elsewhere because their reliability has not been sufficiently established in a systematic (scientific) manner in accordance with the principles discussed in Chapter 4. The chapter focuses primarily on the forensic science disciplines’ capability for providing evidence that can be presented in court. As such, there is considerable discussion about the reliability and precision of results—attributes that factor into probative value and admissibility decisions. It should be recalled, however, that forensic science also provides great value to law enforcement investigations, and even those forensic science disciplines whose scientific foundation is currently limited might have the capacity (or the potential) to provide probative information to advance a criminal investigation.1 This chapter also provides the committee’s summary assessment of each of these disciplines.2 1 For example, forensic odontology might not be sufficiently grounded in science to be admissible under Daubert, but this discipline might be able to reliably exclude a suspect, thereby enabling law enforcement to focus its efforts on other suspects. And forensic science methods that do not meet the standards of admissible evidence might still offer leads to advance an investigation. 2 The chapter does not discuss eyewitness identification or line-ups, because these techniques do not normally rely on forensic scientists for analysis or implementation. They clearly are of major importance for investigations and trials, and their effective use and interpretation relies on scientific knowledge and continuing research. For similar reasons, this chapter does not delve into the polygraph. The validity of polygraph testing for security screening was addressed
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Strengthening Forensic Science in the United States: A Path Forward Because forensic science aims to glean information from a wide variety of clues and evidence associated with a crime, it deals with a broad range of tools and with evidence of highly variable quality. In general, the forensic science disciplines are pragmatic, with practitioners adopting, adapting, or developing whatever tools and technological aids they can to distill useful information from crime scene evidence. Many forensic science methods have been developed in response to such evidence—combining experience-based knowledge with whatever relevant science base exists in order to create a procedure that returns useful information. Although some of the techniques used by the forensic science disciplines—such as DNA analysis, serology, forensic pathology, toxicology, chemical analysis, and digital and multimedia forensics—are built on solid bases of theory and research, many other techniques have been developed heuristically. That is, they are based on observation, experience, and reasoning without an underlying scientific theory, experiments designed to test the uncertainties and reliability of the method, or sufficient data that are collected and analyzed scientifically. In the course of its deliberations, the committee received testimony from experts in many forensic science disciplines concerning current practices, validity, reliability and errors, standards, and research.3 From this testimony and from many written submissions, as well as from the personal experiences of the committee members, the committee developed the consensus views presented in this chapter. BIOLOGICAL EVIDENCE Biological evidence is provided by specimens of a biological origin that are available in a forensic investigation. Such specimens may be found at the scene of a crime or on a person, clothing, or weapon. Some—for example, pet hairs, insects, seeds, or other botanical remnants—come from the crime scene or from an environment through which a victim or suspect has recently traversed. Other biological evidence comes from specimens obtained directly from the victim or suspect, such as blood, semen, saliva, vaginal secretions, sweat, epithelial cells, vomitus, feces, urine, hair, tissue, bones, and microbiological and viral agents. The most common types of biological evidence collected for examination are blood, semen, and saliva. Human biological evidence that contains nuclear DNA can be particularly valuable because the possibility exists to associate that evidence with one individual with a degree of reliability that is acceptable for criminal justice. in National Research Council, Committee to Review the Scientific Evidence on the Polygraph. 2003. The Polygraph and Lie Detection. Washington, DC: The National Academies Press. It does not cover forensic pathology, because that field is addressed in Chapter 9. 3 A complete list of those who provided testimony to the committee is included in Appendix B.
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Strengthening Forensic Science in the United States: A Path Forward Sample Data and Collection At the crime scene, biological evidence is located, documented, collected, and preserved for subsequent analysis in the crime laboratory. Locating and recognizing biological evidence can be more difficult than a layperson would presume. For example, blood is not always red, some red substances are not blood, and most biological evidence, such as saliva or semen, is not readily visible. Crime scene investigators locate biological evidence through tests that screen for the presence of a particular biological fluid (e.g., blood, semen, saliva), and investigators have a choice of techniques.4 For blood they might use an alternate light source (ALS) at 415nm, the wavelength under which bloodstains absorb light and are thus more visible to the naked eye. Most commonly, though, the screening test for blood is a catalytic chemical test that turns color or luminesces in the presence of blood. Scene investigators may also use Luminol, fluorescein, or crystal violet to identify areas at the scene where attempts were made to clean a bloody crime scene. These tests for blood may also locate other evidence that should be collected and taken to the laboratory for analysis. Recently, immunological tests that can identify human hemoglobin or glycophorin A have become available. These are blood-specific proteins that can be demonstrated to be of human origin. At some point in the future, these immunological tests may replace standard chemical tests, and, although more expensive, they are more specific because they identify blood conclusively instead of just presumptively. Investigators also have several techniques for locating semen at the crime scene. Commonly they rely on an ALS, under which semen, other biological fluids, and some other evidence will luminesce. More recently, immunological tests can be used to identify seminal plasma proteins, for example, prostate specific antigen (p30 or PSA) or semenogelin.5 Finding saliva at the scene is mostly happenstance. Although it luminesces with the ALS at specific wavelengths, the glow is not as strong, and a weaker ALS light source may not highlight it well and possibly not at all. Thus, it can be easily missed. Screening tests for saliva are chemical tests that identify amylase, an enzyme occurring in high concentrations in saliva. But the screening is not definitive, because other types of tissue also 4 Interpreting the results of any screening test requires expertise and experience. Many crime scene investigators have the requisite experience, but they may lack a scientific background, and it is not always straightforward to correctly interpret the results of screening tests. Crime scene investigations that require science-based screening tools are most reliable if someone is involved who understands the physics and chemistry of those tools. 5 I. Sato, M. Sagi, A. Ishiwari, H. Nishijima, E. Ito, and T. Mukai. 2002. Use of the “SMITEST” PSA card to identify the presence of prostate-specific antigen in semen and male urine. Forensic Science International 127(1-2):71-74.
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Strengthening Forensic Science in the United States: A Path Forward contain amylase, including the particular type (AMY 1) that is associated with saliva. Analyses Although the forensic use of nuclear DNA is barely 20 years old, DNA typing is now universally recognized as the standard against which many other forensic individualization techniques are judged. DNA enjoys this preeminent position because of its reliability and the fact that, absent fraud or an error in labeling or handling, the probabilities of a false positive are quantifiable and often miniscule. However, even a very small (but nonzero) probability of false positive can affect the odds that a suspect is the source of a sample with a matching DNA profile.6 The scientific bases and reliability of other types of biological analysis are also well established, but absent nuclear DNA, they can only narrow the field of suspects, not suggest any particular individual. Testing biological evidence in the laboratory involves the use of a logical sequence of analyses designed to identify what a substance is and then from whom it came. The sequence begins with a forensic biologist locating the substance on the evidence. This is followed by a presumptive test that would give more information about the substance, typically using the same tests employed by scene investigators: the ALS, enzymatic, chemical, or immunological tests. Once the material (e.g., blood, semen, or saliva) is known, an immunological test or a human DNA test is run to determine whether the sample comes from a human or an animal. The final step in the analytical sequence procedure is to identify the source of the biological material. If a sufficient sample is present and is probative, the forensic biologist prepares the material for DNA testing. The analyst who conducts the DNA test may or may not be the same person who examines the original physical evidence, depending on laboratory policies. A decision might be required regarding the type of DNA testing to employ. Two primary types of DNA tests are conducted in U.S. forensic laboratories: nuclear testing and mitochondrial DNA (mtDNA) testing, with several variations of the former. For most biological evidence having evidentiary significance, forensic DNA laboratories employ nuclear testing routinely,7 and testing for the 13 core Short Tandem Repeat (STR) 6 W.C. Thompson, F. Taroni, and C.G.G. Aitken. 2003. How the probability of a false positive affects the value of DNA evidence. Journal of Forensic Sciences 48(1):47-54. 7 T.R. Moretti, A.L. Baumstark, D.A. Defenbaugh, K.M. Keys, J.B. Smerick, and B. Budowle B. 2001. Validation of short tandem repeats (STRs) for forensic usage: Performance testing of fluorescent multiplex STR systems and analysis of authentic and simulated forensic samples. Journal of Forensic Sciences 46(3):647-660.
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Strengthening Forensic Science in the United States: A Path Forward polymorphisms is the first line of attack.8 The results are entered into the Federal Bureau of Investigation’s (FBI’s) Combined DNA Indexing System (CODIS) and are searched against DNA profiles already in one of three databases: a convicted felon database, a forensic database containing DNA profiles from crime scenes, and a database of DNA from unidentified persons. Sometimes the evidence dictates testing just for Y STRs, which assesses only the Y (male) chromosome. In sexual assaults for which only small amounts of male nuclear DNA are available (e.g., a large excess of vaginal DNA), it is possible to obtain a Y STR profile of the male who left the semen. Unlike the 13 core loci used in CODIS searches, where a match of all 13 is a strong indicator that both samples come from the same individual, Y STR testing is not as definitive with respect to identifying a single person. A third nuclear test involves the analysis of single nucleotide polymorphisms (SNPs). Although no public forensic DNA laboratory in the United States is routinely analyzing forensic evidence for SNPs, the utility of this genomic information for cases in which the DNA is too damaged to allow standard testing has garnered attention since its use in the World Trade Center identification effort.9 If insufficient nuclear DNA is present for STR testing, or if the existing nuclear DNA is degraded, two options potentially are available. One technique amplifies the amount of DNA available, although this technique is not widely available in U.S. forensic laboratories. A second alternative is to sequence mitochondrial DNA (mtDNA). Since 1996, it has been possible to compare single-source crime scene samples and samples from the victim or defendant on the basis of mtDNA. Four FBI-supported mtDNA laboratories and a few private mtDNA laboratories conduct DNA casework. This technique has been particularly helpful with regard to hairs—which do not contain enough nuclear DNA to enable analysis with current methods unless the root is present—and bones and teeth. Because it measures only a single locus of the genome, mtDNA analysis is much less discriminating than nuclear DNA analysis; all people with a common female ancestor (within the past few generations) share a common profile. But mtDNA testing has forensic value in its ability to include or exclude an individual as its source. Laboratories entering the results of forensic DNA testing into CODIS must meet specific quality guidelines, which include the requirement that 8 Some laboratories are now using 16 loci, 13 of which are the original core loci. 9 B. Leclair, R. Shaler, G.R. Carmody, K. Eliason, B.C.Hendrickson, T. Judkins, M.J. Norton, C. Sears, and T. Scholl. 2007. Bioinformatics and human identification in mass fatality incidents: The World Trade Center disaster. Journal of Forensic Sciences 52(4):806-819. Epub May 25, 2007.
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Strengthening Forensic Science in the United States: A Path Forward the laboratory be accredited and that specific procedures be in place and followed. In accredited laboratories, forensic DNA personnel must take proficiency tests and must meet specific educational and training requirements. (See Chapter 8 for further discussion.) Laboratory analyses are conducted by scientists with degrees ranging from a bachelor’s degree in science to a doctoral degree. Each forensic DNA laboratory has a technical leader, who normally must meet additional experience and educational requirements. Although DNA laboratories are expected to conduct their examinations under stringent quality controlled environments, errors do occasionally occur. They usually involve situations in which interpretational ambiguities occur or in which samples were inappropriately processed and/or contaminated in the laboratory. Errors also can occur when there are limited amounts of DNA, which limits the amount of test information and increases the chance of misinterpretation. Casework reviews of mtDNA analysis suggest a wide range in the quality of testing results that include contamination, inexperience in interpreting mixtures, and differences in how a test is conducted.10 Reporting of Results FBI quality guidelines require that reports from forensic DNA analysis must contain, at a minimum, a description of the evidence examined, a listing of the loci analyzed, a description of the methodology, results and/or conclusions, and an interpretative statement (either quantitative or qualitative) concerning the inference to be drawn from the analysis.11 10 Personal communication, Terry Melton, Mitotyping Laboratory. December 2007. See also L. Prieto; A. Alonso; C. Alves; M. Crespillo; M. Montesino; A. Picornell; A. Brehm; J.L. Ramirez; M.R. Whittle; M.J. Anjos; I. Boschi; J. Buj; M. Cerezo; S. Cardoso; R. Cicarelli; D. Comas; D. Corach; C. Doutremepuich; R.M. Espinheira; I. Fernandez-Fernandez; S. Filippini; Julia Garcia-Hirschfeld; A. Gonzalez; B. Heinrichs; A. Hernandez; F.P.N. Leite; R.P. Lizarazo; A.M. Lopez-Parra; M. Lopez-Soto; J.A. Lorente; B. Mechoso; I. Navarro; S. Pagano; J.J. Pestano; J. Puente; E. Raimondi; A. Rodriguez-Quesada; M.F. Terra-Pinheiro; L. Vidal-Rioja; C. Vullo; A. Salas. 2008. GEP-ISFG collaborative exercise on mtDNA: Reflections about interpretation, artefacts and DNA mixtures. Forensic Science International: Genetics 2(2):126-133; and A. Salas, L. Prieto, M. Montesino, C. Albarrán, E. Arroyo, M. Paredes-Herrera, A. Di Lonardo, C. Doutremepuich, I. Fernández-Fernández, A. de la Vega. 2005. Mitochondrial DNA error prophylaxis: Assessing the causes of errors in the GEP’02-03 proficiency testing trial. Forensic Science International 148(2-3):191-198. 11 DNA Advisory Board. 2000. Quality assurance standards for forensic DNA testing laboratories. Forensic Science Communications 2(3). Available at www.bioforensics.com/conference04/TWGDAM/Quality_Assurance_Standards_2.pdf.
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Strengthening Forensic Science in the United States: A Path Forward Summary Assessment Unlike many forensic techniques that were developed empirically within the forensic science community, with limited foundation in scientific theory or analysis, DNA analysis is a fortuitous by-product of cutting-edge science. Eminent scientists contributed their expertise to ensuring that DNA evidence offered in a courtroom would be valid and reliable (e.g., in the 1989 New York case, People v. Castro), and by 1996 the National Academy of Sciences had convened two committees that issued influential recommendations on handling DNA forensic science.12 As a result, principles of statistics and population genetics that pertain to DNA evidence were clarified, the methods for conducting DNA analyses and declaring a match became less subjective, and quality assurance and quality control protocols were designed to improve laboratory performance. DNA analysis is scientifically sound for several reasons: (1) there are biological explanations for individual-specific findings; (2) the 13 STR loci used to compare DNA samples were selected so that the chance of two different people matching on all of them would be extremely small; (3) the probabilities of false positives have been explored and quantified in some settings (even if only approximately); (4) the laboratory procedures are well specified and subject to validation and proficiency testing; and (5) there are clear and repeatable standards for analysis, interpretation, and reporting. DNA analysis also has been subjected to more scrutiny than any other forensic science discipline, with rigorous experimentation and validation performed prior to its use in forensic investigations. As a result of these characteristics, the probative power of DNA is high. Of course, DNA evidence is not available in every criminal investigation, and it is still subject to errors in handling that can invalidate the analysis. In such cases, other forensic techniques must be applied. The probative power of these other methods can be high, alone or in combination with other evidence. This power likely can be improved by strengthening the methods’ scientific foundations and practice, as has occurred with forensic DNA analysis. ANALYSIS OF CONTROLLED SUBSTANCES The term “illicit drugs” is widely used to describe abused substances. Other terms that are used include “abused drugs,” “illegal drugs,” “street drugs,” and, in the United States, “controlled substances.” The latter term refers specifically to drugs that are controlled by federal and state laws.13 12 National Research Council. 1992. DNA Technology in Forensic Science. Washington, DC: National Academy Press; National Research Council. 1996. The Evaluation of Forensic DNA Evidence: An Update. Washington, DC: National Academy Press. 13 See, e.g., 21 U.S.C.A. § 802(6).
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Strengthening Forensic Science in the United States: A Path Forward The analysis of controlled substances is a mature forensic science discipline and one of the areas with a strong scientific underpinning. The analytical methods used have been adopted from classical analytical chemistry, and there is broad agreement nationwide about best practices.14 In 1997, the U.S. Drug Enforcement Administration and the Office of National Drug Control Policy co-sponsored the formation of the Technical Working Group for the Analysis of Seized Drugs, now known as the Scientific Working Group for the Analysis of Seized Drugs (SWGDRUG). This organization brings together more than 20 forensic practitioners from all over the world to develop standards for the analysis and reporting of illicit drug cases. Their standards are being widely adopted by drug analysis laboratories in the United States and worldwide. Sample Data and Collection Controlled substances typically are seized by police officers, narcotics agents, and detectives through undercover buys, raids on drug houses and clandestine drug laboratories, and seizures on the streets. In some cases, forensic chemists are sent to clandestine laboratory operations to help render the laboratory safe and help with evidence collection. The seized drugs may be in the form of powders or adulterated powders, chunks of smokeable or injectable material, legitimate and clandestine tablets and capsules, or plant materials or plant extracts. Analyses Controlled substances are analyzed by well-accepted standard schemes or protocols. Few drug chemists have the requisite botanical background to identify any common illicit plants other than marijuana; thus, in cases that require botanical identification, the assistance of outside experts is enlisted. Sampling can be a major issue in the analysis of controlled substances. Although sometimes only trace amounts of a drug are present (e.g., in a syringe used to inject heroin), at other times there are hundreds or thousands of packages of drugs or very large bags or bales. SWGDRUG and others have proposed statistical and nonstatistical methods for sampling,15 and a wide variety of methods are used. Most controlled substances are subjected first to a field test for pre- 14 See F. Smith and J.A. Siegel (eds.). 2004. Handbook of Forensic Drug Analysis. Burlington, MA: Academic Press. 15 Scientific Working Group for the Analysis of Seized Drugs (SWGDRUG) Recommendations. Available at www.swgdrug.org/approved.htm.
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Strengthening Forensic Science in the United States: A Path Forward sumptive identification. This is followed by gas chromatography-mass spectrometry (GC-MS), in which chromatography separates the drug from any diluents or excipients, and then mass spectrometry is used to identify the drug. This is the near universal test for identifying unknown substances. Marijuana is an exception, because it is identified normally through a sequence of tests—a presumptive color test, followed by low-powered microscopic identification, and finally by thin-layer chromatography. Reporting of Results Most drug chemists produce terse reports for attorneys and courts. The reports contain administrative data and a short description of the evidence. The weight or number of exhibits is stated and then the results of the analysis. A typical report for a marijuana case might read as follows: Received: Item 1—a sealed plastic bag containing 25.6 g of green-brown plant material. Results: The green-brown plant material in item 1 was identified as marijuana. Some laboratories might mention the tests that were conducted, but in most cases the spectra, chromatograms, and other evidence of the analysis and the chemist’s notes are not submitted. Likewise, possible sources of error and statistical data are not commonly included. From a scientific perspective, this style of reporting is often inadequate, because it may not provide enough detail to enable a peer or other courtroom participant to understand and, if needed, question the sampling scheme, process(es) of analysis, or interpretation. Summary Assessment The chemical foundations for the analysis of controlled substances are sound, and there exists an adequate understanding of the uncertainties and potential errors. SWGDRUG has established a fairly complete set of recommended practices.16 It also provides pointers to a number of guidelines for statistical sampling, both for illegal drugs per se (created by the European Network of Forensic Science Institutes) and for materials more generally (created by the American Society for Testing and Materials). The SWGDRUG recommendations include a menu of analytical chemistry techniques that are considered acceptable in certain circumstances. Because this menu was constructed to be applicable worldwide, it includes 16 See www.swgdrug.org/approved.htm.
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Strengthening Forensic Science in the United States: A Path Forward options that allow laboratories to substitute a concatenation of simple methods if they do not have access to the preferred analytical equipment (e.g., GC-MS). It is questionable, however, whether all of the possible combinations recommended by SWGDRUG would be acceptable in a scientific sense, if one’s goal were to identify and classify a completely unknown substance. The committee has been told that experienced forensic chemists and good forensic laboratories understand which tests (or combinations of tests) provide adequate reliability, but the SWGDRUG recommendations do not ensure that these tests will be used. This ambiguity would be a less significant issue if the reports presented in court contained sufficient detail about the methods of analysis. FRICTION RIDGE ANALYSIS Fingerprints, palm prints, and sole prints have been used to identify people for more than a century in the United States. Collectively, the analysis of these prints is known as “friction ridge analysis,” which consists of experience-based comparisons of the impressions left by the ridge structures of volar (hands and feet) surfaces. Friction ridge analysis is an example of what the forensic science community uses as a method for assessing “individualization”—the conclusion that a piece of evidence (here, a pattern left by friction ridges) comes from a single unambiguous source. Friction ridge analysis shares similarities with other experience-based methods of pattern recognition, such as those for footwear and tire impressions, toolmarks, and handwriting analysis, all of which are discussed separately below. Friction ridge analysis is performed in various settings, including accredited crime laboratories and nonaccredited facilities. Nonaccredited facilities may be crime laboratories, police “identification units,” or private practice (consultants). In some instances, the latent print examiner is employed solely to perform latent print casework. Some examiners may also perform other types of forensic casework (e.g., footwear and tire impressions, firearms analysis). In some agencies, fingerprint examiners also are required to respond to crime scenes and can be sworn officers who also perform police officer/detective duties. The training of personnel to perform latent print identifications varies from agency to agency. Agencies may have a formalized training program, may use an informal mentoring process, or may send new examiners to a one- to two-week course. The International Association for Identification (IAI) offers a training publication, “Friction Ridge Skin Identification Training Manual,”17 and the Scientific Working Group on Friction Ridge 17 International Association for Identification. Friction Ridge Skin Identification Training Manual. Available at www.theiai.org.
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Strengthening Forensic Science in the United States: A Path Forward Analysis, Study and Technology (SWGFAST) offers a guideline, “Training to Competency for Latent Print Examiners.”18 Although these are excellent resources, they are not required, and there is no auditing of the content of training programs developed by nonaccredited agencies. The IAI also offers a certification test that measures both the knowledge and skill of latent print examiners; however, not all agencies require latent print examiners to achieve and maintain certification. Method of Data Collection and Analysis The technique used to examine prints made by friction ridge skin is described by the acronym ACE-V: “Analysis, Comparison, Evaluation, and Verification.”19 It has been described in forensic literature as a means of comparative analysis of evidence since 1959.20 The process begins with the analysis of the unknown friction ridge print (now often a digital image of a latent print). Many factors affect the quality and quantity of detail in the latent print and also introduce variability in the resulting impression. The examiner must consider the following: Condition of the skin—natural ridge structure (robustness of the ridge structure), consequences of aging, superficial damage to the skin, permanent scars, skin diseases, and masking attempts. Type of residue—natural residue (sweat residue, oily residue, combinations of sweat and oil); other types of residue (blood, paint, etc.); amount of residue (heavy, medium, or light); and where the residue accumulates (top of the ridge, both edges of the ridge, one edge of the ridge, or in the furrows). Mechanics of touch—underlying structures of the hands and feet (bone creates areas of high pressure on the surface of the skin); flexibility of the ridges, furrows, and creases; the distance adjacent ridges can be pushed together or pulled apart during lateral movement; the distance the length of a ridge might be compressed or stretched; the rotation of ridge systems during torsion; and the effect of ridge flow on these factors. Nature of the surface touched—texture (rough or smooth), flexibility (rigid or pliable), shape (flat or curved), condition (clean or dirty), and background colors and patterns. 18 SWGFAST. 2002. Training to Competency for Latent Print Examiners. Available at www.SWGFAST.org. 19 Ashbaugh, op. cit.; Triplette and Cooney, op. cit.; J. Vanderkolk. 2004. ACE-V: A model. Journal of Forensic Identification 54(1):45-52; SWGFAST. 2002. Friction Ridge Examination Methodology for Latent Print Examiners. Available at www.SWGFAST.org. 20 R.A. Huber. 1959-1960. Expert witness. Criminal Law Quarterly 2:276-296.
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Strengthening Forensic Science in the United States: A Path Forward sis of intact explosives, with the macroscopic and microscopic analysis of the evidence submitted (whether it is an expended device, fragments of a device, or debris from near the site of the explosion). If no intact explosive material is found, a sequence of extracts may be used to capture any organic and/or inorganic residues present. These extracts are then analyzed employing the same instrumentation used for intact explosives. However, the results produced differ in their specificity, and it is here that the training and expertise of the examiner plays a large role. To interpret the results properly, the examiner must have knowledge of the composition of explosives and the reaction products that form when they explode. Interpretation can be further complicated by the presence of contaminants from, for example, the device or soil.116 Examination conclusions for postblast residues range from “the residue present was consistent with an explosive material” to “the residue is only indicative of an explosive” to “no explosive residues were present.” TWGFEX recently has developed a set of guidelines for the analysis of postblast explosive residues,117 but has yet to make any recommendations for report wording. The examination of fire debris not associated with explosions often aims to determine whether an accelerant was used. To assess the effects of an accelerant, one might design an experiment, under a range of conditions (e.g., wind speed, temperature, presence/absence of other chemicals) with two groups: one in which materials are burned in the presence of an accelerant (“treatment”) and one with no accelerant (“control”). The measured outcomes on the burned materials might be measures that characterize the damage patterns (e.g., depth of char, size of bubbles on surfaces). Differences in the ranges of these measurements from the materials in the two groups (treatment versus control) suggest a hypothesis about the effects of an accelerant. Following this exploration, one should design validation studies to confirm that these measures do indeed characterize the differences in materials treated or untreated with an accelerant. Summary Assessment The scientific foundations exist to support the analysis of explosions, because such analysis is based primarily on well-established chemistry. As part of the laboratory work, an analyst often will try to reconstruct the bomb, which introduces procedural complications, but not scientific ones. 116 C.R. Midkiff. 2002. Arson and explosive investigation. In: R. Saferstein (ed.). Forensic Science Handbook. Vol. 1, 2nd ed. Upper Saddle River, NJ: Prentice Hall. 117 TWGFEX Recommended Guidelines for Forensic Identification of Post-Blast Explosive Residues. 2007. Available at http://ncfs.ucf.edu/twgfex/action_items.html.
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Strengthening Forensic Science in the United States: A Path Forward By contrast, much more research is needed on the natural variability of burn patterns and damage characteristics and how they are affected by the presence of various accelerants. Despite the paucity of research, some arson investigators continue to make determinations about whether or not a particular fire was set. However, according to testimony presented to the committee,118 many of the rules of thumb that are typically assumed to indicate that an accelerant was used (e.g., “alligatoring” of wood, specific char patterns) have been shown not to be true.119 Experiments should be designed to put arson investigations on a more solid scientific footing. FORENSIC ODONTOLOGY Forensic odontology, the application of the science of dentistry to the field of law, includes several distinct areas of focus: the identification of unknown remains, bite mark comparison, the interpretation of oral injury, and dental malpractice. Bite mark comparison is often used in criminal prosecutions and is the most controversial of the four areas just mentioned. Although the identification of human remains by their dental characteristics is well established in the forensic science disciplines, there is continuing dispute over the value and scientific validity of comparing and identifying bite marks.120 Many forensic odontologists providing criminal testimony concerning bite marks belong to the American Board of Forensic Odontology (ABFO), which was organized in 1976 and is recognized by the American Academy of Forensic Sciences as a forensic specialty. The ABFO offers board certification to its members.121 Sample Data and Collection Bite marks are seen most often in cases of homicide, sexual assault, and child abuse. The ABFO has approved guidelines for the collection of evidence from bite mark victims and suspected biters.122 The techniques for obtaining bite mark evidence from human skin—for example, various forms of photography, dental casts, clear overlays, computer enhancement, electron microscopy, and swabbing for serology or DNA—generally are 118 J. Lentini. Scientific Fire Analysis, LLC. Presentation to the committee. April 23, 2007. Available at www7.nationalacademies.org/stl/April%20Forensic%20Lentini.pdf. 119 NFPA 921 Guide for Explosion and Fire Investigations, 2008 Edition. Quincy, MA: National Fire Protection Association. 120 E.g., J.A. Kieser. 2005. Weighing bitemark evidence: A postmodern perspective. Journal of Forensic Science, Medicine, and Pathology 1(2):75-80. 121 American Board of Forensic Odontology at www.abfo.org. 122 Ibid.
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Strengthening Forensic Science in the United States: A Path Forward well established and relatively noncontroversial. Unfortunately, bite marks on the skin will change over time and can be distorted by the elasticity of the skin, the unevenness of the surface bite, and swelling and healing. These features may severely limit the validity of forensic odontology. Also, some practical difficulties, such as distortions in photographs and changes over time in the dentition of suspects, may limit the accuracy of the results.123 Analyses The guidelines of the ABFO for the analysis of bite marks list a large number of methods for analysis, including transillumination of tissue, computer enhancement and/or digitalization of the bite mark or teeth, stereomicroscopy, scanning electron microscopy, video superimposition, and histology.124 The guidelines, however, do not indicate the criteria necessary for using each method to determine whether the bite mark can be related to a person’s dentition and with what degree of probability. There is no science on the reproducibility of the different methods of analysis that lead to conclusions about the probability of a match. This includes reproducibility between experts and with the same expert over time. Even when using the guidelines, different experts provide widely differing results and a high percentage of false positive matches of bite marks using controlled comparison studies.125 No thorough study has been conducted of large populations to establish the uniqueness of bite marks; theoretical studies promoting the uniqueness theory include more teeth than are seen in most bite marks submitted for comparison. There is no central repository of bite marks and patterns. Most comparisons are made between the bite mark and dental casts of an individual or individuals of interest. Rarely are comparisons made between the bite mark and a number of models from other individuals in addition to those of the individual in question. If a bite mark is compared to a dental cast using the guidelines of the ABFO, and the suspect providing the dental cast cannot be eliminated as a person who could have made the bite, there is no established science indicating what percentage of the population or subgroup of the population could also have produced the bite. This follows from the basic problems inherent in bite mark analysis and interpretation. As with other “experience-based” forensic methods, forensic odontology suffers from the potential for large bias among bite mark experts in evaluating a specific bite mark in cases in which police agencies provide the suspects for comparison and a limited number of models from which 123 Rothwell, op. cit. 124 American Board of Forensic Odontology, op. cit. 125 Bowers, op. cit.
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Strengthening Forensic Science in the United States: A Path Forward to choose from in comparing the evidence. Bite marks often are associated with highly sensationalized and prejudicial cases, and there can be a great deal of pressure on the examining expert to match a bite mark to a suspect. Blind comparisons and the use of a second expert are not widely used. Scientific Interpretation and Reporting of Results The ABFO has issued guidelines for reporting bite mark comparisons, including the use of terminology for conclusion levels, but there is no incentive or requirement that these guidelines be used in the criminal justice system. Testimony of experts generally is based on their experience and their particular method of analysis of the bite mark. Some convictions based mainly on testimony by experts indicating the identification of an individual based on a bite mark have been overturned as a result of the provision of compelling evidence to the contrary (usually DNA evidence).126 More research is needed to confirm the fundamental basis for the science of bite mark comparison. Although forensic odontologists understand the anatomy of teeth and the mechanics of biting and can retrieve sufficient information from bite marks on skin to assist in criminal investigations and provide testimony at criminal trials, the scientific basis is insufficient to conclude that bite mark comparisons can result in a conclusive match. In fact, one of the standards of the ABFO for bite mark terminology is that, “Terms assuring unconditional identification of a perpetrator , or without doubt, are not sanctioned as a final conclusion.”127 Some of the basic problems inherent in bite mark analysis and interpretation are as follows: The uniqueness of the human dentition has not been scientifically established.128 The ability of the dentition, if unique, to transfer a unique pattern to human skin and the ability of the skin to maintain that uniqueness has not been scientifically established.129 The ability to analyze and interpret the scope or extent of distortion of bite mark patterns on human skin has not been demonstrated. The effect of distortion on different comparison techniques is not fully understood and therefore has not been quantified. 126 Bowers, op. cit. 127 American Board of Forensic Odontology, op. cit. 128 Senn, op. cit. 129 Ibid.
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Strengthening Forensic Science in the United States: A Path Forward A standard for the type, quality, and number of individual characteristics required to indicate that a bite mark has reached a threshold of evidentiary value has not been established. Summary Assessment Despite the inherent weaknesses involved in bite mark comparison, it is reasonable to assume that the process can sometimes reliably exclude suspects. Although the methods of collection of bite mark evidence are relatively noncontroversial, there is considerable dispute about the value and reliability of the collected data for interpretation. Some of the key areas of dispute include the accuracy of human skin as a reliable registration material for bite marks, the uniqueness of human dentition, the techniques used for analysis, and the role of examiner bias.130 The ABFO has developed guidelines for the analysis of bite marks in an effort to standardize analysis,131 but there is still no general agreement among practicing forensic odontologists about national or international standards for comparison. Although the majority of forensic odontologists are satisfied that bite marks can demonstrate sufficient detail for positive identification,132 no scientific studies support this assessment, and no large population studies have been conducted. In numerous instances, experts diverge widely in their evaluations of the same bite mark evidence,133 which has led to questioning of the value and scientific objectivity of such evidence. Bite mark testimony has been criticized basically on the same grounds as testimony by questioned document examiners and microscopic hair examiners. The committee received no evidence of an existing scientific basis for identifying an individual to the exclusion of all others. That same finding was reported in a 2001 review, which “revealed a lack of valid evidence to support many of the assumptions made by forensic dentists during bite mark comparisons.”134 Some research is warranted in order to identify the circumstances within which the methods of forensic odontology can provide probative value. 130 Ibid. 131 American Board of Forensic Odontology, op. cit. 132 I.A. Pretty. 2003. A Web-based survey of odontologists’ opinions concerning bite mark analyses. Journal of Forensic Sciences 48(5):1-4. 133 C.M. Bowers. 2006. Problem-based analysis of bite mark misidentifications: The role of DNA. Forensic Science International 159 Supplement 1:s104-s109. 134 I.A. Pretty and D. Sweet. 2001. The scientific basis for human bitemark analyses—A critical review. Science and Justice 41(2):85-92. Quotation taken from the abstract.
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Strengthening Forensic Science in the United States: A Path Forward BLOODSTAIN PATTERN ANALYSIS Understanding how a particular bloodstain pattern occurred can be critical physical evidence, because it may help investigators understand the events of the crime. Bloodstain patterns occur in a multitude of crime types—homicide, sexual battery, burglary, hit-and-run accidents—and are commonly present. Bloodstain pattern analysis is employed in crime reconstruction or event reconstruction when a part of the crime scene requires interpretation of these patterns. However, many sources of variability arise with the production of bloodstain patterns, and their interpretation is not nearly as straightforward as the process implies. Interpreting and integrating bloodstain patterns into a reconstruction requires, at a minimum: an appropriate scientific education; knowledge of the terminology employed (e.g., angle of impact, arterial spurting, back spatter, castoff pattern); an understanding of the limitations of the measurement tools used to make bloodstain pattern measurements (e.g., calculators, software, lasers, protractors); an understanding of applied mathematics and the use of significant figures; an understanding of the physics of fluid transfer; an understanding of pathology of wounds; and an understanding of the general patterns blood makes after leaving the human body. Sample Data and Collection Dried blood may be found at crime scenes, deposited either through pooling or via airborne transfer (spatter). The patterns left by blood can suggest the kind of injury that was sustained, the final movements of a victim, the angle of a shooting, and more. Bloodstains on artifacts such as clothing and weapons may be crucial to understanding how the blood was deposited, which can indicate the source of the blood. For example, a stain on a garment, such as a shirt, might indicate contact between the person who wore the shirt and a bloody object, while tiny droplets of blood might suggest proximity to a violent event, such as a beating. Analyses Bloodstain patterns found at scenes can be complex, because although overlapping patterns may appear simple, in many cases their interpreta-
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Strengthening Forensic Science in the United States: A Path Forward tions are difficult or impossible.135,136 Workshops teach the fundamentals of basic pattern formation and are not a substitute for experience and experimentation when applying knowledge to crime reconstruction.137 Such workshops are more aptly applicable for the investigator who needs to recognize the importance of these patterns so that he or she may enlist the services of a qualified expert. These courses also are helpful for attorneys who encounter these patterns in the course of preparing a case or when preparing to present testimony in court. Although there is a professional society of bloodstain pattern analysts, the two organizations that have or recommend qualifications are the IAI and the Scientific Working Group on Bloodstain Pattern Analysis (SWGSTAIN). SWGSTAIN’s suggested requirements for practicing bloodstain pattern analysis are outwardly impressive, as are IAI’s 240 hours of course instruction. But the IAI has no educational requirements for certification in bloodstain pattern analysis.138 This emphasis on experience over scientific foundations seems misguided, given the importance of rigorous and objective hypothesis testing and the complex nature of fluid dynamics. In general, the opinions of bloodstain pattern analysts are more subjective than scientific. In addition, many bloodstain pattern analysis cases are prosecution driven or defense driven, with targeted requests that can lead to context bias. Summary Assessment Scientific studies support some aspects of bloodstain pattern analysis. One can tell, for example, if the blood spattered quickly or slowly, but some experts extrapolate far beyond what can be supported. Although the trajectories of bullets are linear, the damage that they cause in soft tissue and the complex patterns that fluids make when exiting wounds are highly variable. For such situations, many experiments must be conducted to determine what characteristics of a bloodstain pattern are caused by particular actions during a crime and to inform the interpretation of those causal links and 135 H.L. MacDonell. 1997. Bloodstain Patterns. Corning, NY: Laboratory of Forensic Science; S. James. 1998. Scientific and Legal Applications of Bloodstain Pattern Interpretation. Boca Raton, FL: CRC Press; P. Pizzola, S. Roth, and P. DeForest. 1986. Blood drop dynamics–II. Journal of Forensic Sciences 31(1): 36-49. 136 Ibid.; R.M. Gardner. 2004. Practical Crime Scene Processing and Investigation. Boca Raton, FL: CRC Press; H.C. Lee; T. Palmbach and M.T. Miller. 2005. Henry Lee’s Crime Scene Handbook. Burlington, MA: Elsevier Academic Press, pp. 281-298. 137 W.J. Chisum and B.E. Turvey. 2007. Crime Reconstruction. Burlington, MA: Elsevier Academic Press. 138 See “Bloodstain Pattern Examiner Certification Requirements.” Available at theiai.org/certifications/bloodstain/requirements.php.
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Strengthening Forensic Science in the United States: A Path Forward their variabilities. For these same reasons, extra care must be given to the way in which the analyses are presented in court. The uncertainties associated with bloodstain pattern analysis are enormous. AN EMERGING FORENSIC SCIENCE DISCIPLINE: DIGITAL AND MULTIMEDIA ANALYSIS The analysis of digital evidence deals with gathering, processing, and interpreting digital evidence, such as electronic documents, lists of phone numbers and call logs, records of a device’s location at a given time, e-mails, photographs, and more. In addition to traditional desktop and laptop computers, digital devices that store data of possible value in criminal investigations include cell phones, GPS devices, digital cameras, personal digital assistants (PDAs), large servers and storage devices (e.g., RAIDS and SANS), video game consoles (e.g., PlayStation and Xbox), and portable media players (e.g., iPods). The storage media associated with these devices currently fall into three broad categories. The first, magnetic memory, includes hard drives, floppy discs, and tapes. The second, optical memory, includes compact discs (CDs), and digital versatile discs (DVDs). The third, electrical storage, includes USB flash drives, some memory cards, and some microchips. These items are the most commonly encountered in criminal and counterintelligence matters, but laboratories have been asked to examine such items as scuba dive watches in death investigations and black boxes in aircraft mishaps. The proliferation of computers and related devices over the past 30 years has led to significant changes in and the expansion of the types of criminal activities that generate digital evidence. Initially, computers were either the weapon or the object of the crime. In the early days, most computer crime involved manipulating computer programs of large businesses in order to steal money or other resources. As computers became more popular, they became storage containers for evidence. Drug dealers, book makers, and white collar criminals began to keep computerized spreadsheets detailing their transactions. Digital cameras and the Internet have made child pornography increasingly available, and computers act as a digital file cabinet to hold this contraband material. Finally, digital media have become witnesses to daily activities. Many individuals have two cell phones with text messaging and/or e-mail capability, several computers, a home alarm system, a GPS in the car, and more; even children often possess some subset of these items. Workplaces use magnetic card readers to permit access to buildings. Most communication involves some kind of computer, and by the end of each day, hundreds of megabytes of data may have been generated about where individuals have been, how fast they got there, to whom they spoke, and even what was said. Suicide notes are written on
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Strengthening Forensic Science in the United States: A Path Forward computers. Sexual predators stalk their victims online via e-mail, chat, and instant messaging. Even get-away cars are equipped with GPS devices. Finally, computer systems have become (with ever-increasing frequency) the victims of unauthorized control or intrusions. These intrusions often result in the manipulation of files and the exfiltration of sensitive information. In addition, computers in automobiles that track speed, breaking, and turning are valuable in accident reconstruction. As a result, almost every crime could have digital evidence associated with it. Sample Data and Collection The best practices for the collection of digital evidence most often call for the person at the scene to disconnect the power cord for the computer and related peripheral equipments (e.g., monitor, printer) and seize these items, as well as any loose storage media such as thumb drives and CDs. This method works well in most cases. However, some data (like recently typed passwords, malicious programs, and active communication programs) are volatile and are stored in the electronic chips of the system. In these circumstances, this information is lost when the device is turned off. In intrusion investigations or in cases in which encryption software is being used, this volatile information could be the key to a successful analysis and prosecution.139 Recognizing potential sources of digital evidence is also an ongoing challenge. Investigators are likely to seize a desktop computer but walk past a PlayStation. Thumb drives can be fashioned to look like a pocket knife, writing pen, or even a piece of sushi. Cell phones and wireless Internet capability present another challenge: If these devices are turned on while in law enforcement custody, they could be remotely accessed and altered by a suspect. Analyses The typical approach to examining a computer involves two main phases. The first is the imaging phase. During this process, the storage device (most often a hard drive) is fitted with an appliance that prevents any new information from being written. Then, all of the data are copied to a new blank hard drive. The copy is compared with the original, most often by using a mathematical algorithm called Message Digest–5, otherwise known as MD5 Hash. The MD5 Hash value gives a unique series of numbers and letters for every file. In the examination phase, this forensi- 139 See W.G. Kruse and J.G. Heiser. 2001. Computer Forensics: Incident Response Essentials. Boston: Addison-Wesley.
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Strengthening Forensic Science in the United States: A Path Forward cally sound copy is examined for saved computer files with probative value. These so-called logical files often are pictures, documents, spreadsheets, and e-mail files that have been saved by the user in various folders or directories. Logical files are patent evidence. Next, the forensic copy is examined for files that have previously been deleted. The computer files are sometimes called physical, because the data are physically present on the hard drive but they are not logically available to the computer operating system. Such files constitute latent evidence. Finally, system files that are created and saved by the operating system are examined. These files are analogous to a surveillance tape that shows programs that were running on the computer and files that were changed. The goal of most of these examinations is to find files with probative information and to discover information about when and how these files came to be on the computer.140 Digital evidence has undergone a rapid maturation process. This discipline did not start in forensic laboratories. Instead, computers taken as evidence were studied by police officers and detectives who had some interest or expertise in computers. Over the past 10 years, this process has become more routine and subject to the rigors and expectations of other fields of forensic science. Three holdover challenges remain: (1) the digital evidence community does not have an agreed certification program or list of qualifications for digital forensic examiners; (2) some agencies still treat the examination of digital evidence as an investigative rather than a forensic activity; and (3) there is wide variability in and uncertainty about the education, experience, and training of those practicing this discipline. A publication of the Department of Justice Computer Crime and Intellectual Property Section, Searching and Seizing Computers and Obtaining Electronic Evidence in Criminal Investigations,141 describes the challenging legal issues surrounding the examination of digital evidence. For example, sometimes the courts have viewed computers as a piece of evidence that is sent to a laboratory for forensic examination, and as having no special legal constraints, while other times, the courts have viewed computers as a virtual room or filing cabinet.142 For the latter cases, a warrant must be 140 See E. Casey. 2004. Digital Evidence and Computer Crime. San Diego, CA: Academic Press; E. Casey. 2001. Handbook of Computer Crime Investigation: Forensic Tools & Technology. San Diego, CA: Academic Press; B. Carrier. 2005. File System Forensic Analysis. Boston: Addison-Wesley; S. Anson and S. Bunting. 2007. Mastering Windows Network Forensics and Investigation. Indianapolis: Sybex; and H. Carvey and D. Kleiman. 2007. Windows Forensic Analysis. Burlington: Syngress. 141 Available at www.usdoj.gov/criminal/cybercrime/s&smanual2002.htm. 142 See, e.g., G.R. McLain, Jr., 2007. United States v. Hill: A new rule, but no clarity for the rules governing computer searches and seizures. George Mason Law Review 14(4):1071-1104; D. Regensburger, B. Bytes, and B. Bonds. 2007. An exploration of the law concerning
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Strengthening Forensic Science in the United States: A Path Forward obtained that specifies how the examination will be conducted and which files can be recovered before the electronic device can be examined. Finally, the analysis of digital evidence differs from other forensic science disciplines because the examination generates not only a forensic report, but also brings to light documents, spreadsheets, and pictures that may have probative value. Different agencies have handled these generated files in different ways: Some treat them as exhibits, while others treat them as derivative evidence that requires a chain of custody and special protection. A growing number of colleges and universities offer courses in computer security and computer forensics. Still, most law enforcement agencies are understaffed in trained computer security experts. CONCLUSIONS The term “forensic science” encompasses a broad range of disciplines, each with its own set of technologies and practices. Wide variability exists across forensic science disciplines with regard to techniques, methodologies, reliability, error rates, reporting, underlying research, general acceptability, and the educational background of its practitioners. Some of the forensic science disciplines are laboratory based (e.g., nuclear and mitochondrial DNA analysis, toxicology, and drug analysis); others are based on expert interpretation of observed patterns (e.g., fingerprints, writing samples, toolmarks, bite marks, and specimens such as fibers, hair, and fire debris). Some methods result in class evidence and some in the identification of a specific individual—with the associated uncertainties. The level of scientific development and evaluation varies substantially among the forensic science disciplines. the search and seizure of computer files and an analysis of the Ninth Circuit’s decision in United States v. Comprehensive Drug Testing, Inc. Journal of Criminal Law and Criminology 97(4)1151-1208.