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Terrorism: Explosives Threat
Ronald L. Simmons
Naval Surface Warfare Center
INTRODUCTION
Material presented at this workshop is based on two previous National Re-
search Council (NRC) and National Academy of Sciences (NAS) committee
studies on bombings. The first study was devoted to explosive materials (exclud-
ing smokeless and black powders) in 1997, prompted by two mass bombings:
the World Trade Center and the Oklahoma City Federal Building.i The second
study, released in 1998, was devoted specifically to smokeless and black pow-
der, which are commonly used in pipe bombs and small improvised explosive
devices.2 Further insight into terrorist attacks abroad has been provided by the
Bremer Commission report, which was issued in 2000.3
Description of Explosive Threat
While it is recognized that the attacks of September 11, 2001, are attributed
to terrorists, this paper is limited to a discussion of threats from explosives.
Within the United States. The explosive threat by terrorist groups has
been rare within the United States. There have been only two bombings in the
country that can be attributed to terrorist groups the mass bombings referred to
above. The first targeted the World Trade Center in New York City on February
26, 1993, leaving 6 people dead and approximately 700 injured. The terrorist
group was Muslim extremists, and the explosive used was homemade nitrated
urea. The amount of explosive involved is unknown but is believed to have been
at least 100 kg.
171
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HIGH-IMPACT TERRORISM
The second mass bombing in the United States occurred at the Oklahoma
City Federal Building on April 19, 1995, and produced 168 fatalities and a large
number of injuries. This action has been attributed to a small group of domestic
terrorists (fewer than four individuals) who used approximately 2200 kg of home-
made ammonium nitrate fuel oil (ANFO) explosive.
On July 20,1996, during the summer Olympic Games, an individual planted
a pipe bomb (actually three bombs taped together) in Centennial Park, Atlanta,
Georgia. Although the bomb was discovered, it was inadvertently detonated be-
fore it could be disarmed. There was one fatality resulting from a heart attack,
not directly from the explosion, and approximately 110 injuries. This bombing
has not been attributed to a terrorist group. The amount of explosive involved is
estimated to be about 1 kg.
Outside the United States. All known bombings occurring outside the
United States and directed against U.S. citizens have been by terrorist groups.
The most recent case was the attack against the USS Cole in the Port of Aden,
Yemen, on October 12, 2000. There were 17 fatalities and approximately 36
injured. It is believed the attack was carried out by a terrorist group of Muslims
using approximately 300 kg of Comp C4 high explosive.
Previous bombings outside the United States include the Planet Hollywood
restaurant in Cape Town, South Africa, on August 25, 1998, where there were 2
fatalities and 25 injured. The bombing has also been tentatively attributed to a
Muslim group of terrorists. The amount of explosive used is unknown but esti-
mated to be around 50 kg.
On August 7, 1998, there were two almost simultaneous explosions at the
U.S. Embassies in Nairobi, Kenya, and Dar Es Salaam, Tanzania. Both have
been tied to Muslim terrorist groups. The total number of fatalities in the two
Embassy bombings has been estimated to be 224, with more than 4600 injured.
The amount and nature of the explosives used are unknown but believed to have
been at least 100 kg.
Earlier notable mass bombings against U.S. citizens abroad include the fol-
lowing:
· On December 21, 1988, an explosion destroyed PanAm Flight 103 over
Lockerbie, Scotland, killing 270 as a result of the explosion of a bomb contain-
ing less than a kilogram of high explosive believed to be Semtex. The incident
has been attributed to a terrorist Muslim group.
· On October 23, 1983, an explosion demolished the U.S. Marines bar-
racks in Beirut, Lebanon, killing approximately 240. The explosion was believed
to have been caused by compressed flammable gas, not high explosives. The
incident was attributed to a terrorist Muslim group.
· On April 18,1983, there was an explosion at the U.S. Embassy in Beirut,
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EXPLOSIVES TERRORISM
173
Lebanon, killing 63. This incident, like the previous terrorist incidents, was at-
tributed to a terrorist Muslim group.
BOMBINGS: DESCRIPTION
Nature of Explosives Used. Aside from the bombings outside the United
States, and neglecting the two mass bombings at the World Trade Center and
Oklahoma City as aberrations, according to statistics kept by the Bureau of A1-
cohol, Tobacco, and Firearms (ATF) and the Federal Bureau of Investigation
(FBI), the explosive materials used in bombings in the United States (in the
decade before 1997) could be defined or categorized as follows:
· 32 percent of the bombings used smokeless or black powder;
.
29 percent used simple chemical mixtures (defined as simple gas-produc-
ing chemicals confined in a container capable of withstanding some pressure
before bursting);
· 16 percent were commercial fireworks or pyrotechnic compositions sim-
ilar to those used in fireworks;
· 3 percent were high explosives or ammonium nitrate (AN) blasting
agents;
.
1 percent were improvised explosive mixtures;
· 14 percent could not be identified; and
· the remaining 5 percent were not reported.
The bombs that were either homemade or of undetermined origin totaled 49
percent, while 33 percent were smokeless, black powder, high explosives, or AN
blasting agents. The remaining 16 percent used fireworks or similar pyrotechnic
mixtures (other than black powder).
There are several likely sources of explosives used in illegal bombings.
They can be made from commonly available chemicals, meaning that a home-
made bomb could be derived from a number of possible chemicals. The explo-
sives could also be bought on the open market, which means they most likely
will be commercial explosives used for legal purposes such as mining. In addi-
tion, the materials could be stolen from either commercial or military sources,
provided by a friend (or friendly country), or obtained from materials removed
from demilitarized warheads or other munitions.
Nature of Bombings. Bombings in the United States can generally be
described as a large number of small bombs causing little damage, punctuated by
infrequent large-scale bombings causing serious consequences and galvanizing
public concern. With the exception of the two large bombings (World Trade
Center and Oklahoma City), bombings in the United States have been largely
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HIGH-IMPACT TERRORISM
confined to attacks on individuals. Exceptions are churches, public places, abor-
tion clinics, and transportation (aircraft). The amount of explosive contained in a
bomb is small, usually less than 1 kg. Alhough the total number of these small
bombings (usually referred to as pipe bombs) averages about 2000 per year,
most are of the nuisance-type firecracker-in-a-mailbox variety. The number of
"significant" bombings (defined as capable of causing serious injury, death, or
more than $1,000 worth of damage) averages about 300 per year, resulting in
about 10 fatalities per year. Of these fatalities, 60 percent are the bomb makers
themselves.
Two federal agencies gather statistics on bombings the Treasury Depart-
ment through the ATE and the Justice Department through the FBI. Each main-
tains separate statistics by means of distributing separate reporting forms to local
law enforcement agencies, who in turn fill out the forms on a voluntary basis.
Discrepancies exist between the two sources of information, and the previous
NRC-NAS committee studies in 1997 and 1998 recommended better coordina-
tion between the two agencies.
Bomb Makers. In a typical year (excluding the two large-scale bombings),
in 76% of the cases where the bomb maker could be identified, the perpetrators
were juveniles. For bombs containing simple chemical mixtures, juveniles were
implicated in 92% of the cases. For bombs containing high explosives, 41%
were made by juveniles, indicating that the explosives were most likely stolen.
The remaining perpetrators were for the most part acquaintances, neighbors, and
domestic partners.
Commercial Explosives. The annual production of high explosives (and
ammonium nitrate) made for legal commercial uses is estimated to be 3 x 109 kg.
In addition, another 10 x 106 kg of smokeless and black powder and 5 x 106 kg
of energetic chemicals and fuels are produced in the United States. By compari-
son, the amount used in illegal bombings is extremely small, and policy mea-
sures to address this illegal use impinge on the substantially larger legitimate use
of these materials in everyday commerce. The disruptive effect on this everyday
commerce would have to be addressed.
THREAT PREVENTION
To control the illegal use of explosives and reduce (or prevent) the threat,
the two previous NRC-NAS committee studies focused on four areas:
1. Detecting bombs or explosives and preventing explosions;
2. Identifying the source of the explosives to prosecute offenders and dis-
courage other potential bomb makers;
3. Employing additives to render explosives or explosive precursors inert; and
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EXPLOSIVES TERRORISM
175
4. Imposing stricter regulatory controls on explosives and explosive precur-
sors to make it more difficult to obtain or make illegal explosives.
lowing:
Detection. Likely scenarios for detecting explosives/bombs include the fol-
1. Portal scenario, in which people and packages pass through a well-
monitored checkpoint. In such a case, the bomb must be detected with a high
degree of reliability and with a very low false alarm rate. Also, such portal
monitoring strongly discourages would-be bomb makers, since they know that it
will be virtually impossible to pass through undetected.
2. Suspicious package scenario, in which a suspicious package is discov-
ered that may or may not contain a bomb. The problem here is to quickly detect
whether or not a bomb is present.
3. Bomb threat scenario, in which there is reason to believe a bomb exists
somewhere within a large expanse. This problem is the most difficult largely due
to the very short time and very large expanse in which the bomb must be found
before it is likely to explode.
Bombs and explosives can be detected by exploiting properties of both the
explosive and the container. In the case of explosives that do not detonate, they
must be packaged in a container that acts as a pressure vessel, rupturing only
after a certain pressure has been developed. Such containers are usually heavy-
wall metal pipes, which may be covered with fragmenting materials such as nails
and tacks. Thus, the container provides certain characteristics that make it sus-
ceptible to detection.
Current x-ray systems can detect bombs at a portal, entryway, or passage-
way, but they are not suitable for large numbers of packages, large-area search-
es, or in situations where the location of the bomb is unknown.
Canines. Some explosives for example, nitroglycerin dynamites contain
volatile components that are detectable by dogs. These volatile components rep-
resent a wide bouquet of odors, and it is not known specifically what it is that the
dogs sniff. Canines are most successful and reliable in detecting explosive mate-
rials, according to the U.S. Secret Service. Both canine and human search is the
only viable means of conducting large-area searches for hidden bombs. Unfortu-
nately, circumstances exist in which there is chemical interference with canine
detection, and the interference is not well understood.
Chemical Markers. It has been noted that chemical markers could assist in
the detection of explosives or bombs, especially in large-area searches, suspi-
cious packages, and rapid and routine screening of large numbers of packages,
and they would enhance canine sniffing capability.
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HIGH-IMPACT TERRORISM
Chemical markers have been found to be very effective in plastic and sheet
explosives that detonate in small diameters and thicknesses. Such explosives are
CompC4 and Semtex. In 1995, the International Civil Aviation Organization
(ICAO) adopted 2,3-dimethyl-2,3-dinitrobutane (DMNB) for these plastic and
sheet explosives. DMNB has a low vapor pressure and, thus, a reasonably long
shelf life, yet it has sufficient vapor pressure to allow reliable detection. At 25°C,
DMNB has a vapor pressure of 2.1 x 10-3 tort, while the solid explosive RDX
has a vapor pressure of 1.4 x 10-9 tort, and another solid explosive pentaerythri-
tol tetranitrate (PETN) has a vapor pressure of 3.8 x 10-~° torn By comparison,
nitroglycerin (NG) has a vapor pressure of 2.3 x 10~ torn Thus, DMNB is much
more easily detected than the neat explosive itself. Other chemical markers were
evaluated (e.g., ethylene glycol dinitrate [EGDN] and ortho- and para-mononi-
trotoluene), but they were rejected in favor of DMNB because of their higher
volatility and hence shorter shelf life.
DMNB is used at a concentration of 1.0% and costs about $0.40 per pound.
Since Comp C4 costs between $11 and $20 per pound, the addition of DMNB
does not add substantially to the cost of manufacture. On the other hand, the
addition of DMNB to ammonium nitrate (which costs about $0.10 per pound)
increases the cost of AN about 20%, not to mention the problems resulting from
wide distribution of the chemical marker in the environment and the consequent
false alarm rate. The use of AN blasting agents is several orders of magnitude
greater than for plastic and sheet explosives.
The use of DMNB or a similar chemical marker in smokeless and black
powder is not feasible at this time because of concerns of chemical compatibili-
ty, thermal stability, shelf life, and growing ineffectiveness through widespread
usage. The legal use of millions of pounds of chemically marked smokeless and
black powder would contaminate the environment and with time increase the
false positive detection rate. Research has been recommended by the National
Academy of Sciences to find suitable chemical markers in the event that their
use in smokeless and black powder is warranted by increased threat levels.
ID Tagging. Unfortunately, more than 90 percent of the deaths and 80
percent of the injuries from bombs occur in locations with no security screen-
ing that is, no portal screening exists. Fortunately today, forensic evidence
from partially consumed explosives at the bomb scene can usually permit identi-
fication not only of the type of explosive, but also of the manufacturer of that
explosive. However, it is impossible to determine details such as the date of
manufacture and/or lot number. Existing databases of information on smokeless
and black powder are incomplete, although both the ATF and the FBI have about
100 different types and grades of these powders in their databases.
Taggants added to explosive powders could assist a bombing investigation.
The ideal taggant would have the following characteristics:
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EXPLOSIVES TERRORISM
177
No real or perceived health risks;
Wide forensic applicability for law enforcement;
No physical and chemical incompatibility with smokeless or black powder;
No adverse impact on the environment;
Low cost to commercial products;
No viable countermeasures; and
Unique information that is easy to read.
No successful taggant system has been demonstrated to date. All have inherent
limitations. The current record-keeping system does not permit tracing specific
lots or batches of explosive materials from the manufacturer through the complex
distribution system to the final retailer and buyer. Unfortunately, increased record
keeping may increase the number of lawsuits against the manufacturers of legal
explosives and related industries. At this time, the costs outweigh implementation
of taggants, and both NRC committees that studied this problem have recom-
mended more research into taggants, in the event that the threat level increases.
Rendering the Explosive Inert. Ideally, an inerting substance is capable of
preventing an explosion when intimately mixed with other materials chosen to
provide the correct reaction stoichiometry. For example, it may be possible to
mix substance "A" with substance "B" to prevent the use of "B" as an explosive.
It must also prevent detonation in large diameters even when initiated by a large
explosive booster.
In a 1968 U.S. patent, S.J. Porter4 claimed that 10 percent ammonium phos-
phate added to fertilizer-grade AN will produce a non-detonatable mixture when
used in ANFO mixtures. Following the 1995 Oklahoma City bombing, tests
showed that non-detonatable Porter mixtures would actually detonate in charges
where the diameter was 2150 cm or when the charge weight was 236 kg. Tests
also showed that the dilution of AN with 20 percent limestone does not achieve
the desired inerting.
It was concluded that although there are a large number of common chemi-
cals that could be used in bombings, the most likely material is AN. Despite
much research in the United States and abroad, no practical method for inerting
AN has yet been found. Furthermore, no widely accepted test exists for deter-
mining a priori the detonatability of a bulk, improvised AN explosive, such as
the one used in the Oklahoma City bombing.
Limiting Access to Explosives. A very large number of chemicals can be
used to make effective bombs. These are not listed explicitly as explosive mate-
rials, nor are they regulated by federal law. Some of these chemicals are molecu-
lar explosives, which can be used neat, or in mixtures with other reactive materi-
als, or are non-explosive compounds (e.g., oxidizers and fuels), which can be
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HIGH-IMPACT TERRORISM
combined to produce explosive compositions. For explosive purposes, these are
known as precursors. Although the list of possibilities can be quite long, a short
list of most likely precursors for explosive use is as follows:
Explosive Chemicals
Ammonium nitrate
Nitromethane
Oxidizers
Sodium nitrate
Potassium nitrate
Sodium chlorate
Potassium chlorate
Potassium perchlorate
Ammonium perchlorate
Reactant Chemicals
Nitric acid (concentrated)
Hydrogen peroxide (concentrated)
Urea
Inert precursors that can be reacted to produce explosives in one or two steps
include acetone, ammonia, cellulose, glycerin, ethylene glycol, and hexamine.
It is recognized that given the proper expertise and equipment, explosive
compounds can be synthesized from many starting materials, including many
not listed above.5
SUMMARY
Both previous NRC committees that studied the explosive threat problem
came to the following conclusions:
· Compared to many countries, the United States has relatively lax federal
controls on the purchase of explosive materials.
· Many explosives used in bombings are stolen.
.
_
Effective bombs can be made from readily available chemicals.
· It is not feasible to control all possible chemical precursors.
· Criminal access to explosives can be made more difficult through legisla-
tive actions.
· It is not realistic to expect to prevent or deter all illegal bombings.
· A more realistic goal is to make it more difficult for would-be bomb
makers to operate and increase the chances that they will be caught.
· The current level of threat does not warrant additional controls.
.
A major escalation of terrorism might merit emergency controls as a
suitable response.
· At a lower level of threat, the use of such controls would have a severely
disruptive effect on legitimate commercial industries and would be costly.
· Impacted industries include chemicals, explosives, and mining.
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EXPLOSIVES TERRORISM
.
179
Bomb makers have demonstrated that they can change tactics in response
to shifts in controls or availability of chemical precursors.
.
The level of sophistication of bomb makers will likely increase.
U.S. policy makers determine what constitutes a specific level of threat.
Basic to a workable U.S. national strategy regarding the threat from the illegal
use of explosives, smokeless powder, and black powder is the importance of
maintaining a flexible approach to detection markers, identification taggants,
and the regulation of explosives and chemical precursors.
DISCLAIMER
Although the author is employed by the U.S. Navy at the Naval Surface
Warfare Center, Indian Head, Maryland, participation in this workshop is not
under the auspices of the U.S. Navy. The material presented in this workshop is
based on the author's experience in industry and with the National Academy of
Sciences and does not reflect official Navy policy.
NOTES
1. Committee on Marking, Rendering Inert, and Licensing of Explosive Materials, National
Research Council. 1998. Containing the Threat from Illegal Bombings. Washington, D.C.: National
Academy Press.
2. Committee on Smokeless and Black Powder, National Research Council. 1998. Black and
Smokeless Powders Technologies for Finding Bombs and the Bomb Makers. Washington, D.C.:
National Academy Press.
3. National Commission on Terrorism (established by Congress in October 1998), Ambassador
L. Paul Bremer III, Chairman. 2000. Countering the Changing Threat of International Terrorism.
Available on line at: http://www.terrorism.com/documents/bremercommission/index.shtml.
4. Porter, S.J. 1968. Method of Desensitizing Fertilizer Grade Ammonium Nitrate and the
Product Obtained. U.S. Patent 3,366,468
5. Fedoroff, B.T., ed. 1960-1983. Encyclopedia of Explosives and Related Items. Dover, N.J.:
Picatinny Arsenal. Vols. 1-10; Urbanski, T.1964. Chemistry and Technology of Explosives. New
York: Pergamon Press, Vols. 1-3; Davis, T.L. 1943. The Chemistry of Powder and Explosives.
Hollywood, Calif.: Angriff Press.
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Representative terms from entire chapter:
bomb makers
