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Suggested Citation:"2 Precursor Chemicals Used to Make Homemade Explosives." National Academies of Sciences, Engineering, and Medicine. 2017. Reducing the Threat of Improvised Explosive Device Attacks by Restricting Access to Explosive Precursor Chemicals. Washington, DC: The National Academies Press. doi: 10.17226/24862.
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Suggested Citation:"2 Precursor Chemicals Used to Make Homemade Explosives." National Academies of Sciences, Engineering, and Medicine. 2017. Reducing the Threat of Improvised Explosive Device Attacks by Restricting Access to Explosive Precursor Chemicals. Washington, DC: The National Academies Press. doi: 10.17226/24862.
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Suggested Citation:"2 Precursor Chemicals Used to Make Homemade Explosives." National Academies of Sciences, Engineering, and Medicine. 2017. Reducing the Threat of Improvised Explosive Device Attacks by Restricting Access to Explosive Precursor Chemicals. Washington, DC: The National Academies Press. doi: 10.17226/24862.
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Suggested Citation:"2 Precursor Chemicals Used to Make Homemade Explosives." National Academies of Sciences, Engineering, and Medicine. 2017. Reducing the Threat of Improvised Explosive Device Attacks by Restricting Access to Explosive Precursor Chemicals. Washington, DC: The National Academies Press. doi: 10.17226/24862.
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Suggested Citation:"2 Precursor Chemicals Used to Make Homemade Explosives." National Academies of Sciences, Engineering, and Medicine. 2017. Reducing the Threat of Improvised Explosive Device Attacks by Restricting Access to Explosive Precursor Chemicals. Washington, DC: The National Academies Press. doi: 10.17226/24862.
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Page 25
Suggested Citation:"2 Precursor Chemicals Used to Make Homemade Explosives." National Academies of Sciences, Engineering, and Medicine. 2017. Reducing the Threat of Improvised Explosive Device Attacks by Restricting Access to Explosive Precursor Chemicals. Washington, DC: The National Academies Press. doi: 10.17226/24862.
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Page 26
Suggested Citation:"2 Precursor Chemicals Used to Make Homemade Explosives." National Academies of Sciences, Engineering, and Medicine. 2017. Reducing the Threat of Improvised Explosive Device Attacks by Restricting Access to Explosive Precursor Chemicals. Washington, DC: The National Academies Press. doi: 10.17226/24862.
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Page 27
Suggested Citation:"2 Precursor Chemicals Used to Make Homemade Explosives." National Academies of Sciences, Engineering, and Medicine. 2017. Reducing the Threat of Improvised Explosive Device Attacks by Restricting Access to Explosive Precursor Chemicals. Washington, DC: The National Academies Press. doi: 10.17226/24862.
×
Page 28
Suggested Citation:"2 Precursor Chemicals Used to Make Homemade Explosives." National Academies of Sciences, Engineering, and Medicine. 2017. Reducing the Threat of Improvised Explosive Device Attacks by Restricting Access to Explosive Precursor Chemicals. Washington, DC: The National Academies Press. doi: 10.17226/24862.
×
Page 29
Suggested Citation:"2 Precursor Chemicals Used to Make Homemade Explosives." National Academies of Sciences, Engineering, and Medicine. 2017. Reducing the Threat of Improvised Explosive Device Attacks by Restricting Access to Explosive Precursor Chemicals. Washington, DC: The National Academies Press. doi: 10.17226/24862.
×
Page 30
Suggested Citation:"2 Precursor Chemicals Used to Make Homemade Explosives." National Academies of Sciences, Engineering, and Medicine. 2017. Reducing the Threat of Improvised Explosive Device Attacks by Restricting Access to Explosive Precursor Chemicals. Washington, DC: The National Academies Press. doi: 10.17226/24862.
×
Page 31
Suggested Citation:"2 Precursor Chemicals Used to Make Homemade Explosives." National Academies of Sciences, Engineering, and Medicine. 2017. Reducing the Threat of Improvised Explosive Device Attacks by Restricting Access to Explosive Precursor Chemicals. Washington, DC: The National Academies Press. doi: 10.17226/24862.
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Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

2 Precursor Chemicals Used to Make Homemade Explosives The number of precursor chemicals that can be used in the manufacture of homemade explosives (HMEs) is large. To prioritize the precursor chemicals examined throughout the report, the committee compiled a long list of precursor chemicals; then it established a short list of chemicals of particular concern; and lastly, it applied a set of criteria to the chemicals on the short list and, according to those criteria, it ranked the chemicals in three separate groups: A, B, and C. The Group A precursor chemicals are those that the committee determined to pose the most immediate threats in terms of their potential for use in IEDs, though shifts in bomb makers’ tactics could elevate the status of Group B and C chemicals, without warning. PAST AND RECENT ATTACKS INVOLVING EXPLOSIVES The committee produced a list of selected explosives incidents, both realized and thwarted, starting with the 1970 Sterling Hall Bombing at the University of Wisconsin (Table 2-1).34-36 This incident was chosen as a logical starting point as it was the first major attack in the United States that employed precursor chemicals to produce the IED’s main charge, specifically, ammonium nitrate (AN) mixed with fuel oil (AN/FO). The main charge of an IED contains the largest amount of explosive; a description of the main charge utilized in each attack is shown in Table 2-1, along with the estimated mass. The majority of domestic incidents have utilized and continue to utilize commercial explosives, smokeless powder, black powder, flash powder and pyrotechnic fillers as a main charge likely due to their ease of acquisition (e.g. purchasing fifty pounds of black powder requires no federal license or permit).37,38 These materials have been used in high-profile incidents like the Boston Marathon bombing.4 However, as shown in Table 2-1, precursor chemicals have played an important role in many bombing incidents over the past several decades. Events that occurred prior to the Sterling Hall attack primarily relied on commercial explosives (mainly dynamite), with bombers only transitioning to fertilizer and other precursor-based HMEs once commercial explosives became less accessible. PREPUBLICATION COPY: UNCORRECTED PROOFS 21

22 Restrictin Access to Explosive P ing o Precursor C Chemicals T TABLE 2-1 Selected att tacks involvi explosiv from 197 to 2017 ing ves 70 N NOTE: AN: ammonium nitrate, AN m N/FO: ammon nium nitrate//fuel oil, PE ETN: pentaer rythritol tetra anitrate, C CAN: calcium ammoniu nitrate, NM: nitromet um N thane, TATP triacetone triperoxide, BP: black p P: , powder, C CHP: concen ntrated hydro ogen peroxid TNT: trin de, nitrotoluene, IS: icing su , ugar † †upper limit of charge mass. m G Gray: event involving pr i recursor chem micals. Whit event usi ng commerc or milita explosive te: cial ary es. S Stripes: even with ambig nt guous sources. S Appendi C for an expanded tab with boos See ix e ble sters and init tiators. PREPUBLIC P CATION CO OPY: UNCOR RRECTED P PROOFS

Precursor Chemicals Used to Make Homemade Explosives 23 It would be highly impractical to attempt to compile a list of all explosive attacks over nearly 50- year span covered by Table 2-1. The committee chose to highlight the events in the table for one or more of three reasons: • events were either high profile terrorist attacks that garnered appreciable political or public attention, or struck high profile U.S. targets outside active war zones; • events used HMEs; and • events had reliable forensic data with which to identify the charge. In the 1970s, a large number of small dynamite bombs (less than 20 lb) were utilized in the United States. While incidents, such as the Harvey’s Casino bombing, that involved dynamite in larger-scale devices, garnered significant attention at the time, such incidents are not listed in Table 2-1. Moreover, this list also does not reflect the use of IEDs in active military theaters. Between the 1970s and 2000, a series of larger vehicle bombs emerged in terrorist attacks with main charges in the thousands of pounds range, but in the following decade, bombs with smaller charges like those seen in the 1970s started to appear again. By the 2010s, there was a growth towards smaller charges using HMEs. Similarly, there was a related expansion from fertilizer-based materials to a more diverse range of possible precursor chemicals. HMEs are produced either by blending or cooking. Blending is the most common form of manufacture, and the simplest, as it requires only physically mixing the precursor chemicals together. To make a blended explosive, at least one precursor chemical must be an oxidizer (a chemical source of oxygen) and one must be a fuel (a chemical or compound that can react with oxygen in a combustion- like process). The blasting agent AN/FO and flash powder are both examples of blended mixtures. Cooking, a term borrowed from the narcotics enforcement community, is a more complicated manufacturing process to make HMEs wherein multiple precursor chemicals are mixed together and chemically react to form an explosive material. Triacetone triperoxide (TATP), urea nitrate, and ethylene glycol dinitrate (EGDN) are all made through cooking reactions. For many HMEs, more than one synthetic route is possible, involving different precursor chemicals. Groups involved in explosive attacks and the types of explosives employed by each are shown in Figure 2-1. Both the Unabomber39 and the Provisional Irish Republican Army (PIRA)40 represent bombing campaigns with roots traced back to the 1970s, and the Fuerzas Armadas Revolucionarias de Colombia (FARC) has a similarly storied history. The remainder of the groups shown in Figure 2-1 include bomb builders in the Iraq and Afghanistan conflicts as well as the newer factions encountered with the rise of ISIS and other extremists. All of these groups utilize precursor chemicals to produce their HME charges. History has shown that the tactics developed by groups like Al-Qaeda in the Arabian Peninsula have migrated across the world. For example, the trend of using concentrated hydrogen peroxide (CHP) to produce IEDs emerged in Pakistan and rapidly transitioned to Jordan, The United Kingdom, Germany, and, eventually, the United States.41 PREPUBLICATION COPY: UNCORRECTED PROOFS

24 Restrictin Access to Explosive P ing o Precursor C Chemicals F FIGURE 2-1 Terrorist groups and th common used mai charges. g heir nly in Case Study The Evolv y: ving Tactics of a Terro s orist Group The attem to solve a problem by making po mpt b olicy, in the midst of or in response to a crisis, c create can e even greater difficulties. Perhaps one of the best historical ex e xamples of t pitfalls of narrowly f the focusing o immediate events, at least in the context of pr on l c recursor chem micals, is that of the resp ponse of the United e K Kingdom to the explosiv produced by PIRA du ves d uring their bbombing cam mpaign.42 The PIRA bombing campaign be A c egan around 1971 and em mployed dev vices filled w readily a with available d dynamite sto from quarries and mines. In para olen m allel, during this time fra in the U ame United States groups s, s such as Weatther Underground, Fuerz Armadas de Liberación Naciona (FALN), a United F zas s al and Freedom F Front (UFF) also conduc many at cted ttacks using dynamite. R Responding n narrowly to t these events, both t United Kingdom and the United States increa the K d ased control on dynami In the Un ls ite. nited States, b bombers miggrated to read accessib low-expl dily ble losive fillers like black ppowder and s smokeless powder ( (which remain popular choices to thi day). Such materials w not accessible in th United Kin is h were he ngdom, b PIRA wa able to obtain farm ch but as hemicals to replace the dydynamite. The first chemical PI IRA used to produce HM mixtures and replace dynamite w sodium c ME e was chlorate, a strong oxid dizer used as a weed kille Sodium chlorate was mixed with the energet fuel nitrob s er. c s h tic benzene t make sma explosive charges. To counter the threat of ch to all o hlorate explo osives, the Unnited Kingdom government mandated th addition of a diluent to weed kille to reduce i explosive potential. A g he o o er its e After c chlorate was no longer an option, PIR turned to AN. Many farmers in N n RA o y Northern Ireeland possessed large q quantities of AN as it wa a chief fer f as rtilizer found in agricultu In additi d ure. ion, with the heavy equi e ipment r required for farming, ma of the sam farmstea were equ any me ads uipped with ddiesel tanks and pumps. This c combination made for th logical pro he ogression of PIRA devel f loping AN/FFO-based IED Ds. The transsition to AN/FO-based devices by PI d IRA from th earlier dy heir dynamite and chlorate ch d harges h some log had gistical and tactical cons t sequences. Unlike dynam U mite, AN/FO is not cap s O sensitive (the e s sensitivity of an explosiv to initiatio by a #8 de f ve on etonator), do not funct oes tion properly in small ch y harges, a requires some confin and nement to reliably functi As a res ion. sult, the devi ices produce from AN/F ed FO PREPUBLIC P CATION CO OPY: UNCOR RRECTED P PROOFS

Precursor Chemicals Used to Make Homemade Explosives 25 tended to be larger than the previous dynamite and chlorate devices, and often incorporated metal containers to produce greater confinement. The net result was larger, fragment-producing bombs. These larger, heavier IEDs had to be delivered by vehicles due to their mass. Thus, efforts to keep terrorists from accessing dynamite and chlorate resulted in PIRA’s development of the vehicle bomb. The United Kingdom, under pressure to address the trend of vehicle-borne IEDs (VBIEDs), passed legislation in 1972 that outlawed the possession of AN fertilizers that contained more than 27.5% Nitrogen by mass.25 To replace the outlawed AN, farmers selected the fertilizer calcium ammonium nitrate (CAN). CAN consisted of AN combined with dolomitic limestone (a blend of calcium and magnesium carbonate). This mixture contained 21% diluent (by weight) to the 79% AN, and was tested and found incapable of being used to produce AN/FO. It did not take long for PIRA explosive chemists to exploit a simple physical weakness in the new CAN formulation. AN was soluble in water, and the dolomite diluent was not. By mixing the CAN in hot water the AN could be dissolved and separated from the insoluble carbonate component. Once the solid was filtered out, the remaining liquid could be driven off to isolate nearly pure AN. During this time frame United Kingdom authorities came across caches of AN in three purity ranges (100%, 80- 90%, and 60%). It is notable that the 60% AN product was actually more dilute than the CAN the terrorists were trying to pull AN out of. The use of CAN in farming did not stop PIRA, but did make the production of AN-based devices more time consuming and removed the least-adept bomb makers from the picture. Thus, the countermeasure had some limited effect. Initially, the AN recovered from the recrystallization process was not ideal for AN/FO production. It was coarse and crystalline and would not absorb an optimum amount of diesel. To compensate for this change PIRA began using alternative fuels. One very popular formulation developed was a mixture of AN and nitrobenzene (referred to as ANNIE). In 1991, approximately 19 years after its introduction, PIRA discovered that crushing the CAN prills into a powdered form using either industrial strength coffee grinders or barley crushers eliminated the need to isolate purified AN. The pulverized CAN could be mixed with a variety of fuels to make an effective explosive filler. Two fuels surfaced as constants: aluminum powder and powdered (icing) sugar. Aluminum was applied consistently for smaller, mortar-borne charges, and sugar was utilized in the larger-scale VBIEDs. During the 1990s, PIRA perfected the CAN and icing sugar mixture and utilized it in four major bombings in England. Three of these bombs were deployed against the city of London, and one the city of Manchester. The largest was approximately four-thousand pounds (roughly equivalent to the bomb used in Oklahoma City). Parallels exist between PIRA’s development of countermeasures to overcome the United Kingdom’s attempts to regulate the precursor chemicals they were using to make explosives, and explosives produced by the Taliban in Afghanistan in recent years. The Taliban conducted identical processing operations to weaponize AN as did PIRA. However, the Taliban developed their methods in the span of years instead of the decades it took PIRA. PIRA’s transition from commercial dynamite to a variety of AN-based HME mixtures parallels the path of other determined terrorist groups. Initially, groups attempt to procure commercial or military explosives if such are accessible. In the absence of available explosives, they will look for materials that can be blended together, such as AN/FO. Denied the precursors for simple blends they will next resort to processing materials to produce the feedstock of their explosives, such as by isolating AN from CAN. PREPUBLICATION COPY: UNCORRECTED PROOFS

26 Restrictin Access to Explosive P ing o Precursor C Chemicals W each level of difficu introduc into the process, few and fewe bombers w be succe With ulty ced wer er will essful in t their endeavo Howeve any gover ors. er, rnment creatting control for precurso chemicals must take in or nto c consideration the tactics that will be developed in response. n n TIFYING AND PRIOR IDENT A RITIZING P PRECURSO CHEMI OR ICALS USED IN IED ATTA N ACKS Precursor chemicals used to prod r duce IEDs ca be catego an orized by typ and role as: oxidizers, fuels pe , ( (organic mat terials, energ getic organic compounds food produ c s, ucts, or inorg ganic materi ials), and syn nthesis c chemicals (inncluding stro and wea acids, Figu 2-2). Th figure, wh constitut the comm ong ak ure he hich tes mittee’s “ “long list” of precursor chemicals, is not exhaust f c s tive, as it wo ould be impo ossible to list every precu ursor c chemical tha has been or can be used to make an IED. at n F FIGURE 2-2 Long list of precursor chemicals so 2 o orted by che emical type a role. and 2+ + + + 2+ + N NOTE: Ca : calcium; Na : sodium; K : potassium; Ba : ba N ; arium; NH4 : ammonium AN: amm m; monium n nitrate; CAN calcium am N: mmonium ni itrate. PREPUBLIC P CATION CO OPY: UNCOR RRECTED P PROOFS

Precu ursor Chemi icals Used to Make Hom o memade Expl losives 27 Char Size Ana rge alysis Not all pr emicals can be used to make the main charges fo every bom recursor che b m n or mbing scenar rio. F Figure 2-3 suummarizes th various precursor che he p emicals seen as the main charges for different us n n r se-cases: v vehicle-born IEDs (VBIEDs), perso ne on-borne IED (PBIEDs ), aircraft bo Ds ombings, and detonators These d s. a not the on possible charges for each use-ca are nly ase. F FIGURE 2-3 Historical examples of explosive charges and use-cases. 3 f c NOTE: TAT triaceton triperoxide HMTD: hexamethylen triperoxid diamine; EGDN: ethy N TP: ne e; h ne de ylene g glycol dinitra AN: ammonium nitr ate; rate; NM: ni itromethane; CHP: conce ; entrated hyddrogen peroxxide; K KClO3: potas ssium chlora R-salt: cyclotrimethy ate. c ylenetrinitro osamine. Foo products i od include flour and r icing sugar. For a fuller list of food products, ref to Figure 2-2. Fuels in F l p fer nclude diese and saw du el ust. VBIEDs use charges ranging in mass from ap m pproximatel forty poun to tens of thousands of ly nds p pounds, depeending on th carrying capacity of th vehicle. P he he Precursor che emicals utilized to produ these uce e explosives te to be fert end tilizers (e.g., AN and ure potassiu chlorate, and concent ea), um trated hydro ogen p peroxide (CH given th ability to amass these precursor ch HP) he a hemicals in l large quantitties. PBIEDs are typically encountere in backpac brief ca y ed cks, ases, small ba and suic bombin vests, ags, cide ng b belts, etc. Th charge ma of these devices is pr he ass d redicated on what the ind dividual deliivering the ccharge is c capable of caarrying. Hist torically, the charge mas for such P e ss PBIEDs rang from approximately o to ges one f forty pounds PBIEDs ty s. ypically also employ a mass of fragm m mentation ma aterial, such as nails or s screws, t can weig as much as the explos charge itself. that gh a sive i Explosiv used agai aviation targets histo ves inst orically have been milita formulations due to t e ary their r reliability an power, alt nd though, recen terrorist plots against aircraft have started to u HMEs, a nt p e use albeit b below the ma seen in PBIEDs. Ter ass P rrorists use precursor che p emicals freqquently in detonator consstruction, b they also opt for pre- but o -made system acquired from comm ms mercial source when pos es ssible. Deton nators PREPUBLIC P CATION CO OPY: UNCOR RRECTED P PROOFS

28 Restricting Access to Explosive Precursor Chemicals also use precursor chemicals in very small amounts, but the primary explosives they produce are often very sensitive and unstable. Thus, there is an inherent danger in making, handling, transporting, and storing improvised detonators. Due to the lesser orders of magnitude in aviation IED and detonator charge masses—lesser as compared to the VBIEDs and PBIEDs, described above—the committee limited subsequent analysis to those VBIEDs and PBIEDs, both of which entail sufficient risk to merit consideration. A scenario involving a larger-scale VBIED, such as a truck bomb, could entail substantially more damage than a scenario involving a smaller-scale PBIED, such as a back-pack bomb, but be less likely to occur (Appendix B). Thus, the risk of either scenario might rate concern when both dimensions of risk-severity and probability-are included in the assessment. Starting with these scenarios, one can (1) identify the chemicals that terrorists can use to produce each type of device and the conditions under which they can obtain them; (2) develop strategies to reduce the odds of malicious actors getting access to the precursor chemicals; and, (3) ultimately, lessen the risk of either scenario by making both scenarios less likely to happen (i.e., lower probability). While beyond the scope of this study, it may also be possible to drive toward scenarios with less lethal or damaging consequence (i.e., lower severity) by changing access to different precursor chemicals. Generating a Short List of Precursor Chemicals Every exercise in prioritization, including this winnowing process, has an inherent degree of subjectivity. Any one of the precursor chemicals listed in Figure 2-1 could be utilized to produce another devastating attack. To generate a more-focused, short list of precursor chemicals, the committee considered two variables: quantity required and ubiquity. First, it was judged impractical to control very small amounts of any particular precursor chemical and, for this reason, precursor chemicals used only to construct charges for detonators (e.g., mercury and lead azide) and aviation IEDs were eliminated from further consideration. Second, the committee eliminated certain chemicals on the basis of ubiquity. Ubiquity, for the purposes of this study, was used to describe chemicals that are present in high volumes and used in myriad common applications in research, industry, and personal use, such that their analysis by the committee was deemed intractable. All the food products (see Figure 2-2) were removed from consideration because of their ubiquity, as were common hydrocarbons such as diesel fuel. Acetone, however, posed a unique challenge. Acetone is slightly less common than household fuels such as kerosene, but its usage in academia and chemical processes makes it one of the most ubiquitous general solvents in the world. While acetone can be reacted with hydrogen peroxide to produce the explosive TATP, the committee did not include acetone on the short list because it is not considered a threat if appropriate steps are taken to control the peroxide component.14 PREPUBLICATION COPY: UNCORRECTED PROOFS

Precursor Chemicals Used to Make Homemade Explosives 29 Criteria for Generating Groups A, B, and C By removing precursor chemicals used only in very small amounts and ubiquitous materials, such as food products, the committee narrowed the list of chemicals under consideration to just twenty-eight chemicals. To group this short list by priority, the committee adopted three criteria: • the size of the main charge resulting from the precursor chemical, and whether it can be employed in a VBIED, a PBIED, or both; • the history of the precursor chemical’s usage in IED construction; and • whether the precursor chemical can be utilized independently, or is dependent on other precursors listed for the chemical synthesis of an explosive. Under the first criterion, the committee focused on the precursor chemicals that can be used to make HMEs that are suitable for producing either VBIEDs or PBIEDs, with charge sizes ranging from as much as several tons to as little as about a pound, respectively. Some explosives require large masses to propagate a detonation, and the precursors needed to produce these types of materials may not be suited for the production of smaller charges. Other types of explosives are susceptible to initiation of an explosion or detonation from the application of a low-level stimulus, making them either not practical or not easy to make at the hundreds of pounds scale, and the precursors for these sensitive explosives may be limited to utilization in smaller quantities. Limitations of precursor availability also dictates usage and may be independent of the properties of the explosives they can make, as some chemicals are simply not available in large quantities, while others are commonly sold by the ton. Based on all of these factors a precursor may have utility in VBIEDs, PBIEDs, or both. Under the second criterion, past usage of a precursor was taken as an indicator of its continued potential to be applied in IEDs in the present and future. Some precursor chemicals have been consistently utilized in IEDs across the world for many decades, while others have seen only brief usage by one isolated terrorist group or individual, only to quickly disappear from malicious use. Under the third criterion, a precursor chemical merits greater priority if it is independent; that is if the precursor chemical plays an essential part in the synthesis of an explosive material. For example, as seen in Table 2-1 and Figure 2-1, urea nitrate has been used in HMEs in VBIEDs. To synthesize urea nitrate, the precursor chemicals urea and nitric acid are both required; thus, urea nitrate production could be blocked in the absence of either. Of the two, urea is much more commonly available than nitric acid, and the only explosive it can be used to produce is urea nitrate. In contrast, nitric acid can be used to synthesize a variety of other explosive materials. Thus, in this situation, urea would be categorized as dependent (D) on nitric acid, while nitric acid would be judged independent (I). Application of the Criteria to Precursor Chemicals In application, the committee assigned each chemical either higher or lower priority, according to where the chemical stood in relation to each criterion. For the first criterion, chemicals limited to use in either vehicle- or person-borne devices (V or P) were assigned lower priority, while those that could be PREPUBLICATION COPY: UNCORRECTED PROOFS

30 Restricting Access to Explosive Precursor Chemicals reasonably anticipated to produce both VBIEDs and PBIEDs (V/P) were assigned higher priority. Aspects discussed earlier, such as the safety and commonality of the chemicals, were taken into consideration for this analysis (i.e., whether enough of the final main charge explosive material could be assembled from available materials and without killing the bomb maker). For the criterion of historical usage, chemicals previously used to produce explosives (Y) were assigned higher priority, and those whose usage was either extremely rare or largely theoretical (N) were assigned lower priority. Ratings for this criterion introduced an element of professional judgement. Every chemical on the list had been used in a bombing or in IED production in some capacity at least once. Ratings were made in a conservative fashion when possible, with some chemicals that had been used by single groups, under very limiting circumstances, receiving a lower priority rating. In some cases, chemicals that had limited past usage were given a higher priority rating due to their versatility and potential for explosives production. For the third criterion, chemicals judged independent in syntheses (I) were assigned a higher priority, and those judged dependent (D) were assigned a lower priority. In some cases, the committee had to compare a chemical’s global utility to ensure that it rated as dependent for any explosive preparations in which it could be put to use. The committee sorted the chemicals into three groups based on whether they met the conditions of the higher priority for one, two, or three criteria. The committee placed chemicals that met the conditions of a higher priority for three criteria in Group A; for two criteria in Group B; and for one criterion in Group C. The final evaluation is provided in Table 2-2. Coincidently, the precursor chemicals sorted into three groups of almost equal size. In this study, the committee chose to conduct an in-depth examination of the Group A precursor chemicals. The decision to include urea ammonium nitrate (UAN) solution in Group A represents the only departure from a strict application of the committee’s ranking principles. UAN is considered a relatively new product with limited geographical distribution, but commercially available. There is a well- documented history of explosive production from analogous urea-nitrate salt solutions used in Iraq. While UAN has not been used historically to produce explosives, the ease of producing various explosives from nitrating urea solutions, as seen in Iraq, support the notion of UAN as a future threat and justified its inclusion in Group A. There is an additional caveat for certain precursor chemicals insofar as they come in a diverse range of concentrations when contained in commercial products or bulk mixtures. For example, hydrogen peroxide as low as 35% can be quickly blended to make an explosive charge if mixed with the proper fuel. While some control strategies specify concentration thresholds (see Chapter 3 and 4), the lack of a scientific consensus on what those thresholds are, precluded the committee from including concentration thresholds in the prioritized table (Table 2-2). PREPUBLICATION COPY: UNCORRECTED PROOFS

Precu ursor Chemi icals Used to Make Hom o memade Expl losives 31 T TABLE 2-2 Ranking of precursor ch hemicals into three group o ups. N NOTE: *See discussion for explanat e tion of inclu uding UAN in Group A. V: VBIED, P: PBIED, Y used n Y: h historically, N: not used historically, I: independ N dent, D: depe endent. PREPUBLIC P CATION CO OPY: UNCOR RRECTED P PROOFS

32 Restricting Access to Explosive Precursor Chemicals CONCLUSION The National Academies’ 1998 short list, which was later applied by DHS to construct the list of chemicals in CFATS Appendix A, only focused on precursor chemicals that made charges with larger mass sizes suitable for VBIEDs. Looking at the trend in Table 2-1, more bombing incidents are reporting smaller charge mass sizes, consistent with PBIEDs. Based on this trend, the committee chose to look at precursor chemicals that can be used to manufacture VBIEDs or PBIEDs, and further prioritized them using three criteria: suitability for large and small charge size, prior use, and dependency. Every chemical in Table 2-2 is viewed as a viable precursor chemical and a viable threat, whether it has been sorted into Group A, B, or C. Group ranking is somewhat subjective, and could change depending, for example, on the interpretation of existing data or a shift in terrorist tactics. Continuous reevaluation of the precursors is encouraged by the committee, as some of the rankings may change over time with an evolving threat environment. The committee concentrated its efforts on Group A chemicals when examining the supply chains and existing controls, both discussed in Chapter 3. PREPUBLICATION COPY: UNCORRECTED PROOFS

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Improvised explosive devices (IEDs) are a type of unconventional explosive weapon that can be deployed in a variety of ways, and can cause loss of life, injury, and property damage in both military and civilian environments. Terrorists, violent extremists, and criminals often choose IEDs because the ingredients, components, and instructions required to make IEDs are highly accessible. In many cases, precursor chemicals enable this criminal use of IEDs because they are used in the manufacture of homemade explosives (HMEs), which are often used as a component of IEDs.

Many precursor chemicals are frequently used in industrial manufacturing and may be available as commercial products for personal use. Guides for making HMEs and instructions for constructing IEDs are widely available and can be easily found on the internet. Other countries restrict access to precursor chemicals in an effort to reduce the opportunity for HMEs to be used in IEDs. Although IED attacks have been less frequent in the United States than in other countries, IEDs remain a persistent domestic threat. Restricting access to precursor chemicals might contribute to reducing the threat of IED attacks and in turn prevent potentially devastating bombings, save lives, and reduce financial impacts.

Reducing the Threat of Improvised Explosive Device Attacks by Restricting Access to Explosive Precursor Chemicals prioritizes precursor chemicals that can be used to make HMEs and analyzes the movement of those chemicals through United States commercial supply chains and identifies potential vulnerabilities. This report examines current United States and international regulation of the chemicals, and compares the economic, security, and other tradeoffs among potential control strategies.

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