IED Report Highlights 03
Consensus Study Report
November 2017
HigHligHtS
Reducing the Threat of Improvised
Explosive Device Attacks by Restricting
Access to Explosive Precursor Chemicals
A lack of visibility and oversight at the retail level makes it possible for malicious actors to acquire chemicals that could be used to make improvised
explosive devices, this report inds. Bulk quantities of the chemicals, which
have many legitimate uses, are tracked through much of the industrial
supply chain but there is less visibility and oversight as smaller quantities
move further along the supply chain to retail, especially for use by the general
public. This report prioritizes precursor chemicals that can be used to make homemade explosives, examines current domestic and international regulation of the
chemicals, considers the role of voluntary actions, and compares economic, security,
and other tradeoffs among possible control strategies.
An improvised explosive device (IED) is a type of unconventional bomb that can
cause loss of life, injury, and property damage in both military and civilian environments. Terrorists, violent extremists, and criminals (collectively referred to in this
report as malicious actors) often choose IEDs because the ingredients, components,
and instructions required to fabricate them are readily accessible. In many cases,
the explosives in IEDs are made with chemicals that can be purchased legally from
hardware, drug, and garden supply retailers, in brick-and-mortar stores, or online.
How-to guides and videos for making homemade explosives and constructing IEDs
are also widely available and transmitted on the internet.
Precursor chemicals have played an important role in past terrorist attacks in the United
States, some involving large-scale IEDs, such as the truck bombings of the World Trade
Center in 1993 and the Alfred P. Murrah Federal Building in Oklahoma City in 1995, and
others involving smaller-scale, person-borne IEDs. After the Oklahoma City bombing,
Congress placed attention on ammonium nitrate, a chemical principally used as an
agricultural fertilizer, but also used in that attack. Numerous federal agencies (including
the Department of Homeland Security, the Department of Justice, and the Environmental
Protection Agency), state agencies, and voluntary programs managed by private organizations, oversee precursor chemicals in some capacity, but the majority do not focus on
retail-level sales. In 2008, Congress called on the U.S. Department of Homeland Security
to develop a plan to regulate the sale and transfer of ammonium nitrate to prevent its
misappropriation or use in an act of terrorism and in 2011 the Department published a
notice of proposed rulemaking for that purpose that remains under contention. However,
ammonium nitrate is just one of several precursor chemicals that have been used in IEDs.
At the request of the U.S. Department of Homeland Security, the National Academies
of Sciences, Engineering, and Medicine assembled a committee of experts to
consider opportunities to reduce the threat of IED attacks by restricting access to
...deliberative thinking about possible
control strategies is
more efective than
event-driven policy
making in the aftermath of a crisis.
precursor chemicals that can be used to make homemade
explosives. The resulting report prioritizes the precursor
chemicals that can be used to make homemade explosives, analyzes the movement of those chemicals through
the domestic supply chain, and identiies potential
vulnerabilities. The report’s authoring committee examined current domestic and international regulation of
the chemicals, considered the role of voluntary actions,
and compared economic, security, and other tradeoffs
among possible control strategies.
Precursor chemicals used to make
homemade exPlosives
The committee developed a prioritized list of precursor
chemicals according to three criteria: (1) whether the precursor chemical could be used in both vehicle-borne IEDs
and person-borne IEDs, (2) whether the precursor chemical had a history of use in IED attacks, and (3) whether
the precursor chemical can be used to make homemade
explosives independent of the presence of another speciic chemical. On that basis, the committee established
three groups of precursor chemicals—Groups A, B, and
C—by order of priority (see Table 1). With one exception,
chemicals that satisied all three criteria were placed in
Group A, indicating the highest priority. Chemicals that
satisied two of the three criteria were placed in Group
B; and chemicals that satisied one of the three criteria
in Group C. The committee stressed the importance of
reevaluating the groupings periodically to address shifting threats and precursor chemical use.
Binary exploding target kits, which are used as targets for
irearms practice, are also available from brick-and-mortar and online retailers and represent another noteworthy
vulnerability. These kits contain precursor chemicals in
the proper weights and physical forms—and instructions
for use—to create an optimized homemade explosive that
does not require either a commercial detonator or any
other primary explosive to detonate.
international regulations
The report’s authoring committee assessed the regulations in place to restrict access to precursor chemicals in
Australia, Canada, Singapore, the United Kingdom, and
the European Union (EU) for insight into possible control
strategies. Although the beneits of security regulations
are dificult to track, authorities in the EU believe their
Although this report focuses solely on precursor chemicals used to make homemade explosives, the majority of
bombing incidents in the United States involve certain
explosives—such as smokeless powder, black powder,
lash powder, and pyrotechnic illers—likely due to their
ease of legitimate acquisition. Thus, no strategy for
restricting access to precursor chemicals can eliminate
the threat of IED attacks as long as these other explosive
materials remain accessible.
domestic chemical suPPly chains
The available data suggest that a malicious actor can
acquire enough precursor chemicals to manufacture
homemade explosives through legal purchases at retail
outlets. The precursor chemicals sold at retail have legitimate uses, generally fall below regulatory thresholds in
non-agricultural environments, and, as a consequence,
are subject to little or no oversight as a security matter.
The committee concluded that retail-level sales present
a substantial vulnerability in the supply chains under
consideration. Retailers, especially those selling through
internet commerce, have not been a major focus of
federal regulation or of voluntary programs, except
in a limited number of speciic circumstances and
sectors, such as agriculture. Internet commerce presents
additional challenges because of the anonymity of purchasers, ease of sharing information, and large volume
of transactions.
taBle 1 This table presents the committee’s ranking of precursor chemicals into three groups. Notes: V indicates vehicle-borne
IED (~40 lbs to 10,000 or more lbs), P indicates person-borne
IED (~1 lb to ~40 lb), Y (yes) indicates prior use of the chemical in
IEDs, N (no) indicates no prior use in IEDs, I indicates the chemical
can be used for homemade explosives independent of another
speciic chemical, D indicates the precursor chemical requires the
presence of other chemicals to make homemade explosives.
Figure 1 This supply chain diagram shows the movement of precursor chemicals from origin to end use. Chemicals enter the supply chain at blue nodes, are stored at gray nodes (except transportation), and are transformed
into something else or consumed directly at green nodes. Purple indicates the possibility of e-commerce.
restrictions on access, which have included retail bans,
licensing, and registries, have reduced the threat of
attacks using IEDs made with precursor chemicals,
albeit at some cost to commerce.
assessing PossiBle control
strategies
The committee considered four general types of
control strategy, three of which featured a new
control—either a ban, licensing, or a registry—and one
which would not feature a new control, but would
augment any existing controls with supplemental
measures and activities, such as outreach, training,
and reporting.
While the United States has not experienced an attack
with a large-scale IED since the 1990s, the committee stressed that deliberative thinking about possible
control strategies is more effective than event-driven
policy making in the aftermath of a crisis. Using the
lessons learned from international regulations on
precursor chemicals and related domestic policies in
other arenas, the committee assessed possible control
strategies that could include different combinations
of mandatory and voluntary policy mechanisms,
directed at retail sales to noncommercial end-users
(i.e., the general public).
Overall, no single strategy emerged as a clear “best
choice” during the committee’s deliberations on
security, economic, and other tradeoffs. For example,
the committee found the beneits of stringency might
come at the price of forgone sales and use, displacement to other forms of terrorism, and commercial
disruption. The committee lacked the time, resources,
and directive from DHS to conduct a comprehensive
and detailed analysis of policy options. Thus, this
report constitutes a starting point, not an ending point
for evaluating possible control strategies.
The committee considered trade-offs among security, economic, and other factors associated with a
small set of possible control strategies. It assessed
the strategies qualitatively in relation to three goals:
(1) restricting malicious actors’ access to precursor
chemicals, (2) gathering and disseminating information to prevent or respond to terrorist incidents, and
(3) minimizing the burdens on legitimate commerce
and use. In deference to the third goal, the committee
did not consider restrictions on access for commercial
end-users; in each case, a commercial purchaser, such
as a housecleaning, pool, or spa service or beautician,
would only be expected to provide evidence of commercial status to complete a transaction.
recommendations
The report’s authoring committee detailed six recommended courses of action and four research areas that
merit future attention. The committee emphasizes the
essential role of Congress in developing and implementing appropriate risk-reducing control strategies.
Congress could, for example, be particularly instrumental in ensuring that crisis-driven interests do not
unduly inluence new laws or regulations.
Priority Precursor Chemicals
recommendation 1: Federal, state, local, and
private-sector entities attempting to reduce
the threat of IED attacks by restricting access to
precursor chemicals should focus on both personborne and vehicle-borne IEDs.
recommendation 2: Federal, state, local, and
private-sector entities attempting to reduce the
threats from person-borne and vehicle-borne IEDs
should consider multi-chemical, rather than singlechemical, strategies.
Strategies at the Retail Level
recommendation 3: Federal, state, local, and
private-sector entities attempting to reduce the
threats from person-borne and vehicle-borne IEDs
should focus on retail-level transactions of precursor chemicals, especially e-commerce.
recommendation 4: Federal, state, local, and
private-sector entities should explore strategies
for harmonizing oversight of the sale and use of
commercially available kits that contain precursor
chemicals that are speciically designed to be combined to produce homemade explosives.
Analysis of Control Strategies
recommendation 5: U.S. Department of
Homeland Security should engage in a more
comprehensive, detailed, and rigorous analysis of
speciic provisions for proposed mandatory and
voluntary policy mechanisms to restrict access to
precursor chemicals by malicious actors.
Voluntary Measures, Activities, and
Programs
recommendation 6: The federal government
should provide additional support for voluntary
measures, activities, and programs that can contribute to restricting access by malicious actors to
precursor chemicals used to manufacture IEDs.
Research Priorities
In addition to the aforementioned recommendations, the committee identiied several areas of
research that could provide additional pathways
for limiting access to precursor chemicals or
designing appropriate regulations. Major topics
for future research include:
• Standardization of explosive incident data
collection;
• Substitution of precursor chemicals in commercial products;
• Standardization of regulatory thresholds; and
• Understanding behavioral responses, includ-
ing those of terrorists to proposed controls and
those of policy makers to terrorist attacks.
committee on reducing the threat oF imProvised exPlosive device attacks
By restricting access to chemical exPlosive Precursors
victoria a. greenield (Chair), George Mason University; robert g. Best, Defense Threat Reduction Agency –
JIDO; leo e. Bradley, LE Bradley Consulting LLC; John c. Brulia, Austin Powder Company (Retired); carrie l.
castille, Independent Consultant; david g. delaney, University of Maryland; arthur g. Fraas, Resources for
the Future; William J. hurley, Institute for Defense Analysis; karmen n. lappo, Sandia National Laboratories;
Becky d. olinger, Los Alamos National Laboratory; Jimmie c. oxley, University of Rhode Island; kevin F.
smith, Sustainable Supply Chain Consulting; kirk yeager, Federal Bureau of Investigation; camly tran (Study
Director), samuel m. goodman (Postdoctoral Fellow), Jarrett i. nguyen (Senior Program Assistant), National
Academies of Sciences, Engineering, and Medicine
For More Information . . . This Consensus Study Report Highlights was prepared by the Board on Chemical Sciences
and Technology based on the Consensus Study Report Reducing the Threat of Improvised Explosive Device Attacks by
Restricting Access to Explosive Precursor Chemicals (2017). The study was sponsored by the Department of Homeland
Security. Any opinions, indings, conclusions, or recommendations expressed in this publication do not necessarily relect the views of any organization or agency that provided support for the project. Copies of the Consensus
Study Report are available from the National Academies Press, (800) 624-6242; http://www.nap.edu or via the
Board on Chemical Sciences and Technology web page at http://www.nationalacademies.org.
Division on Earth and life Studies
Copyright 2017 by the National Academy of Sciences. All rights reserved.
November 2017
HigHligHtS
Reducing the Threat of Improvised
Explosive Device Attacks by Restricting
Access to Explosive Precursor Chemicals
A lack of visibility and oversight at the retail level makes it possible for malicious actors to acquire chemicals that could be used to make improvised
explosive devices, this report inds. Bulk quantities of the chemicals, which
have many legitimate uses, are tracked through much of the industrial
supply chain but there is less visibility and oversight as smaller quantities
move further along the supply chain to retail, especially for use by the general
public. This report prioritizes precursor chemicals that can be used to make homemade explosives, examines current domestic and international regulation of the
chemicals, considers the role of voluntary actions, and compares economic, security,
and other tradeoffs among possible control strategies.
An improvised explosive device (IED) is a type of unconventional bomb that can
cause loss of life, injury, and property damage in both military and civilian environments. Terrorists, violent extremists, and criminals (collectively referred to in this
report as malicious actors) often choose IEDs because the ingredients, components,
and instructions required to fabricate them are readily accessible. In many cases,
the explosives in IEDs are made with chemicals that can be purchased legally from
hardware, drug, and garden supply retailers, in brick-and-mortar stores, or online.
How-to guides and videos for making homemade explosives and constructing IEDs
are also widely available and transmitted on the internet.
Precursor chemicals have played an important role in past terrorist attacks in the United
States, some involving large-scale IEDs, such as the truck bombings of the World Trade
Center in 1993 and the Alfred P. Murrah Federal Building in Oklahoma City in 1995, and
others involving smaller-scale, person-borne IEDs. After the Oklahoma City bombing,
Congress placed attention on ammonium nitrate, a chemical principally used as an
agricultural fertilizer, but also used in that attack. Numerous federal agencies (including
the Department of Homeland Security, the Department of Justice, and the Environmental
Protection Agency), state agencies, and voluntary programs managed by private organizations, oversee precursor chemicals in some capacity, but the majority do not focus on
retail-level sales. In 2008, Congress called on the U.S. Department of Homeland Security
to develop a plan to regulate the sale and transfer of ammonium nitrate to prevent its
misappropriation or use in an act of terrorism and in 2011 the Department published a
notice of proposed rulemaking for that purpose that remains under contention. However,
ammonium nitrate is just one of several precursor chemicals that have been used in IEDs.
At the request of the U.S. Department of Homeland Security, the National Academies
of Sciences, Engineering, and Medicine assembled a committee of experts to
consider opportunities to reduce the threat of IED attacks by restricting access to
...deliberative thinking about possible
control strategies is
more efective than
event-driven policy
making in the aftermath of a crisis.
precursor chemicals that can be used to make homemade
explosives. The resulting report prioritizes the precursor
chemicals that can be used to make homemade explosives, analyzes the movement of those chemicals through
the domestic supply chain, and identiies potential
vulnerabilities. The report’s authoring committee examined current domestic and international regulation of
the chemicals, considered the role of voluntary actions,
and compared economic, security, and other tradeoffs
among possible control strategies.
Precursor chemicals used to make
homemade exPlosives
The committee developed a prioritized list of precursor
chemicals according to three criteria: (1) whether the precursor chemical could be used in both vehicle-borne IEDs
and person-borne IEDs, (2) whether the precursor chemical had a history of use in IED attacks, and (3) whether
the precursor chemical can be used to make homemade
explosives independent of the presence of another speciic chemical. On that basis, the committee established
three groups of precursor chemicals—Groups A, B, and
C—by order of priority (see Table 1). With one exception,
chemicals that satisied all three criteria were placed in
Group A, indicating the highest priority. Chemicals that
satisied two of the three criteria were placed in Group
B; and chemicals that satisied one of the three criteria
in Group C. The committee stressed the importance of
reevaluating the groupings periodically to address shifting threats and precursor chemical use.
Binary exploding target kits, which are used as targets for
irearms practice, are also available from brick-and-mortar and online retailers and represent another noteworthy
vulnerability. These kits contain precursor chemicals in
the proper weights and physical forms—and instructions
for use—to create an optimized homemade explosive that
does not require either a commercial detonator or any
other primary explosive to detonate.
international regulations
The report’s authoring committee assessed the regulations in place to restrict access to precursor chemicals in
Australia, Canada, Singapore, the United Kingdom, and
the European Union (EU) for insight into possible control
strategies. Although the beneits of security regulations
are dificult to track, authorities in the EU believe their
Although this report focuses solely on precursor chemicals used to make homemade explosives, the majority of
bombing incidents in the United States involve certain
explosives—such as smokeless powder, black powder,
lash powder, and pyrotechnic illers—likely due to their
ease of legitimate acquisition. Thus, no strategy for
restricting access to precursor chemicals can eliminate
the threat of IED attacks as long as these other explosive
materials remain accessible.
domestic chemical suPPly chains
The available data suggest that a malicious actor can
acquire enough precursor chemicals to manufacture
homemade explosives through legal purchases at retail
outlets. The precursor chemicals sold at retail have legitimate uses, generally fall below regulatory thresholds in
non-agricultural environments, and, as a consequence,
are subject to little or no oversight as a security matter.
The committee concluded that retail-level sales present
a substantial vulnerability in the supply chains under
consideration. Retailers, especially those selling through
internet commerce, have not been a major focus of
federal regulation or of voluntary programs, except
in a limited number of speciic circumstances and
sectors, such as agriculture. Internet commerce presents
additional challenges because of the anonymity of purchasers, ease of sharing information, and large volume
of transactions.
taBle 1 This table presents the committee’s ranking of precursor chemicals into three groups. Notes: V indicates vehicle-borne
IED (~40 lbs to 10,000 or more lbs), P indicates person-borne
IED (~1 lb to ~40 lb), Y (yes) indicates prior use of the chemical in
IEDs, N (no) indicates no prior use in IEDs, I indicates the chemical
can be used for homemade explosives independent of another
speciic chemical, D indicates the precursor chemical requires the
presence of other chemicals to make homemade explosives.
Figure 1 This supply chain diagram shows the movement of precursor chemicals from origin to end use. Chemicals enter the supply chain at blue nodes, are stored at gray nodes (except transportation), and are transformed
into something else or consumed directly at green nodes. Purple indicates the possibility of e-commerce.
restrictions on access, which have included retail bans,
licensing, and registries, have reduced the threat of
attacks using IEDs made with precursor chemicals,
albeit at some cost to commerce.
assessing PossiBle control
strategies
The committee considered four general types of
control strategy, three of which featured a new
control—either a ban, licensing, or a registry—and one
which would not feature a new control, but would
augment any existing controls with supplemental
measures and activities, such as outreach, training,
and reporting.
While the United States has not experienced an attack
with a large-scale IED since the 1990s, the committee stressed that deliberative thinking about possible
control strategies is more effective than event-driven
policy making in the aftermath of a crisis. Using the
lessons learned from international regulations on
precursor chemicals and related domestic policies in
other arenas, the committee assessed possible control
strategies that could include different combinations
of mandatory and voluntary policy mechanisms,
directed at retail sales to noncommercial end-users
(i.e., the general public).
Overall, no single strategy emerged as a clear “best
choice” during the committee’s deliberations on
security, economic, and other tradeoffs. For example,
the committee found the beneits of stringency might
come at the price of forgone sales and use, displacement to other forms of terrorism, and commercial
disruption. The committee lacked the time, resources,
and directive from DHS to conduct a comprehensive
and detailed analysis of policy options. Thus, this
report constitutes a starting point, not an ending point
for evaluating possible control strategies.
The committee considered trade-offs among security, economic, and other factors associated with a
small set of possible control strategies. It assessed
the strategies qualitatively in relation to three goals:
(1) restricting malicious actors’ access to precursor
chemicals, (2) gathering and disseminating information to prevent or respond to terrorist incidents, and
(3) minimizing the burdens on legitimate commerce
and use. In deference to the third goal, the committee
did not consider restrictions on access for commercial
end-users; in each case, a commercial purchaser, such
as a housecleaning, pool, or spa service or beautician,
would only be expected to provide evidence of commercial status to complete a transaction.
recommendations
The report’s authoring committee detailed six recommended courses of action and four research areas that
merit future attention. The committee emphasizes the
essential role of Congress in developing and implementing appropriate risk-reducing control strategies.
Congress could, for example, be particularly instrumental in ensuring that crisis-driven interests do not
unduly inluence new laws or regulations.
Priority Precursor Chemicals
recommendation 1: Federal, state, local, and
private-sector entities attempting to reduce
the threat of IED attacks by restricting access to
precursor chemicals should focus on both personborne and vehicle-borne IEDs.
recommendation 2: Federal, state, local, and
private-sector entities attempting to reduce the
threats from person-borne and vehicle-borne IEDs
should consider multi-chemical, rather than singlechemical, strategies.
Strategies at the Retail Level
recommendation 3: Federal, state, local, and
private-sector entities attempting to reduce the
threats from person-borne and vehicle-borne IEDs
should focus on retail-level transactions of precursor chemicals, especially e-commerce.
recommendation 4: Federal, state, local, and
private-sector entities should explore strategies
for harmonizing oversight of the sale and use of
commercially available kits that contain precursor
chemicals that are speciically designed to be combined to produce homemade explosives.
Analysis of Control Strategies
recommendation 5: U.S. Department of
Homeland Security should engage in a more
comprehensive, detailed, and rigorous analysis of
speciic provisions for proposed mandatory and
voluntary policy mechanisms to restrict access to
precursor chemicals by malicious actors.
Voluntary Measures, Activities, and
Programs
recommendation 6: The federal government
should provide additional support for voluntary
measures, activities, and programs that can contribute to restricting access by malicious actors to
precursor chemicals used to manufacture IEDs.
Research Priorities
In addition to the aforementioned recommendations, the committee identiied several areas of
research that could provide additional pathways
for limiting access to precursor chemicals or
designing appropriate regulations. Major topics
for future research include:
• Standardization of explosive incident data
collection;
• Substitution of precursor chemicals in commercial products;
• Standardization of regulatory thresholds; and
• Understanding behavioral responses, includ-
ing those of terrorists to proposed controls and
those of policy makers to terrorist attacks.
committee on reducing the threat oF imProvised exPlosive device attacks
By restricting access to chemical exPlosive Precursors
victoria a. greenield (Chair), George Mason University; robert g. Best, Defense Threat Reduction Agency –
JIDO; leo e. Bradley, LE Bradley Consulting LLC; John c. Brulia, Austin Powder Company (Retired); carrie l.
castille, Independent Consultant; david g. delaney, University of Maryland; arthur g. Fraas, Resources for
the Future; William J. hurley, Institute for Defense Analysis; karmen n. lappo, Sandia National Laboratories;
Becky d. olinger, Los Alamos National Laboratory; Jimmie c. oxley, University of Rhode Island; kevin F.
smith, Sustainable Supply Chain Consulting; kirk yeager, Federal Bureau of Investigation; camly tran (Study
Director), samuel m. goodman (Postdoctoral Fellow), Jarrett i. nguyen (Senior Program Assistant), National
Academies of Sciences, Engineering, and Medicine
For More Information . . . This Consensus Study Report Highlights was prepared by the Board on Chemical Sciences
and Technology based on the Consensus Study Report Reducing the Threat of Improvised Explosive Device Attacks by
Restricting Access to Explosive Precursor Chemicals (2017). The study was sponsored by the Department of Homeland
Security. Any opinions, indings, conclusions, or recommendations expressed in this publication do not necessarily relect the views of any organization or agency that provided support for the project. Copies of the Consensus
Study Report are available from the National Academies Press, (800) 624-6242; http://www.nap.edu or via the
Board on Chemical Sciences and Technology web page at http://www.nationalacademies.org.
Division on Earth and life Studies
Copyright 2017 by the National Academy of Sciences. All rights reserved.