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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