Detection of Explosives on Airline Passengers: Recommendation of the 9/11 Commission and Related Issues
Detection of Explosives on Airline
Passengers: Recommendation of the 9/11
Commission and Related Issues
Summary
Passengers: Recommendation of the 9/11
Commission and Related Issues
Summary
The National Commission on Terrorist Attacks Upon the United States, known as
the 9/11 Commission, recommended that Congress and the Transportation Security
Administration give priority attention to screening airline passengers for explosives.
The key issue for Congress is balancing the costs of mandating passenger explosives
detection against other aviation security needs. Passenger explosives screening
technologies have been under development for several years and are now being deployed
in selected airports. Their technical capabilities are not fully established, and
operational and policy issues have not yet been resolved. Critical factors for
implementation in airports include reliability, passenger throughput, and passenger
privacy concerns. Presuming the successful development and deployment of this
technology, certification standards, operational policy, and screening procedures for
federal use will need to be established. This topic continues to be of congressional
interest, particularly as the 110th Congress reexamines implementation of the 9/11
Commission’s recommendations via H.R. 1 and S. 4.
Introduction
In its discussion of strategies for aviation security, the 9/11 Commission
recommended that:
The TSA [Transportation Security Administration] and the Congress must give
priority attention to improving the ability of screening checkpoints to detect
explosives on passengers. As a start, each individual selected for special screening
should be screened for explosives.
The Intelligence Reform and Terrorism Prevention Act of 2004 (P.L. 108-458)
directed the Department of Homeland Security (DHS) to place high priority on developing
and deploying equipment for passenger explosives screening; required TSA, part of DHS,
to submit a strategic plan for deploying such equipment; and authorized additional
research funding. It also required that passengers who are selected for additional
screening be screened for explosives, as an interim measure until all passengers can be
screened for explosives. Congressional interest in this topic continues, particularly as the
110th Congress reexamines implementation of the 9/11 Commission’s recommendations.
The Implementing the 9/11 Commission Recommendations Act of 2007 (H.R. 1) would
require TSA to issue the strategic plan called for by P.L. 108-458 within seven days of
passage and would establish a Checkpoint Screening Security Fund, paid for with fees on
airline passengers, to develop and deploy equipment for explosives detection at screening
checkpoints. The Improving America’s Security Act of 2007 (S. 4) would require DHS
to issue the same strategic plan within 90 days of passage and begin its implementation
within one year of passage. The U.S. Troop Readiness, Veterans’ Health, and Iraq
Accountability Act, 2007 (H.R. 1591, the FY2007 supplemental appropriations bill)
would provide an additional $45 million for expansion of checkpoint explosives detection
pilot systems. This report discusses the current state of passenger explosives trace
detection and related policy issues.
Current State of Passenger Explosives Trace Detection
Explosives detection for aviation security has been an area of federal concern for
many years. Much effort has been focused on direct detection of explosive materials in
carry-on and checked luggage, but techniques have also been developed to detect and
identify residual traces that may indicate a passenger’s recent contact with explosive
materials. These techniques use separation and detection technologies, such as mass
spectrometry, gas chromatography, chemical luminescence, or ion mobility spectrometry,
to measure the chemical properties of vapor or particulate matter collected from
passengers or their carry-on luggage. Several technologies have been developed and
deployed on a test or pilot basis. Parallel efforts in explosives vapor detection have
employed specially trained animals, usually dogs.
The effectiveness of chemical trace analysis is highly dependent on three distinct
steps: (1) sample collection, (2) sample analysis, and (3) comparison of results with
known standards.2 If any of these steps is suboptimal, the test may fail to detect
explosives that are present. When trace analysis is used for passenger screening,
additional goals may include nonintrusive or minimally intrusive sample collection, fast
sample analysis and identification, and low cost. While no universal solution has yet been
achieved, ion mobility spectrometry is most often used in currently deployed equipment.
In 2004, TSA began pilot projects to deploy portal trace detection equipment for
operational testing and evaluation. In the portal approach, passengers pass through a
device like a large doorframe that can collect, analyze, and identify explosive residues on
the person’s body or clothing. The portal may rely on the passenger’s own body heat to
volatilize traces of explosive material for detection as a vapor, or it may use puffs of air
that can dislodge small particles as an aerosol. Portal deployment is ongoing.3
One alternative to portals is to collect the chemical sample using a handheld vacuum
“wand”. Another is to test an object handled by the passenger, such as a boarding pass,
for residues transferred from the passenger’s hands. In this case, the secondary object is
used as the carrier between the passenger and the analyzing equipment.4
The olfactory ability of dogs is sensitive enough to detect trace amounts of many
compounds, but several factors have inhibited the regular use of canines for passenger
screening. Dogs trained in explosives detection can generally only work for brief periods,
have significant upkeep costs, are unable to communicate the identity of the detected
explosives residue, and require a human handler when performing their detection role.5
In addition, direct contact between dogs and airline passengers raises liability concerns.
Detection of Bulk Explosives
Direct detection of explosives concealed on
passengers in bulk quantities has been another area of federal interest. Technology
development efforts in this area include portal systems based on techniques such as x-ray
backscatter imaging, millimeter wave energy analysis, and terahertz imaging.6 As such
systems detect only bulk quantities of explosives, they would not raise “nuisance alarms”
on passengers who have recently handled explosives for innocuous reasons. Some
versions could simultaneously detect other threats, such as nonmetallic weapons. On the
other hand, trace detection techniques are also likely to detect bulk quantities of
explosives and may alert screening personnel to security concerns about a passenger who
has had contact with explosives but is not actually carrying an explosive device when
screened. Current deployments for passenger screening are focused on trace detection,
and the remainder of this report does not discuss bulk detection. However, many of the
policy issues discussed below would apply similarly to bulk detection equipment.
Policy Issues
Any strategy for deploying and operating passenger explosives detection portals must
consider a number of challenges. Organizational challenges include deciding where and
how detectors are used, projecting costs, and developing technical and regulatory
standards. Operational challenges include maximizing passenger throughput, responding
to erroneous and innocuous detections, ensuring passenger acceptance of new procedures,
minimizing the potential for intentional disruption of the screening process, and providing
for research and development into future generations of detection equipment, including
techniques for detecting novel explosives. For security reasons, many technical details
of equipment performance are not publicly available, which makes independent analysis
of technical performance challenging.
Equipment Location and Use
An important component of a deployment
strategy is identifying where and how passenger explosives detection equipment will be
used. Portals could be deployed widely, so that all locations benefit from them, or they
could be used only at selected locations, where they can most effectively address and
mitigate risk. In any given location, portals could be used as a primary screening
technology for all passengers, or as a secondary screening technology for selected
passengers only. Widespread deployment and use for primary screening might provide
more uniform risk reduction, but would require many more portals and thus increase
costs.
Cost of Operation
The total cost of deploying explosives detection equipment
for passenger screening is unknown. According to TSA, the portal systems currently
being deployed in U.S. airports cost more than $160,000 each.7 Document scanning
systems are somewhat less expensive; according to a 2002 GAO study, similar tabletop
systems used for screening carry-on baggage can cost from $20,000 to $65,000.8 It is
possible that technology improvements or bulk purchasing could lower costs. The
number of devices required would depend on throughput rates, device reliability and
lifetime, and deployment strategy. The United States has more than 400 commercial
passenger airports; if equally distributed, several thousand devices might be required,
corresponding to a total capital cost for equipment of up to hundreds of millions of
dollars. Installation and maintenance costs would be additional. Operating the equipment
would require additional screening procedures and might lead to costs for additional
screening personnel, or else create indirect costs by increasing passenger wait times. It
is unknown whether the personnel limit for TSA screeners, currently set at 45,000 full
time equivalent screeners nationwide (P.L. 108-90), could accommodate the potential
additional staffing requirements.
Standards, Certification, Regulation, and the Establishment of Screening Procedures
Standards for the performance of passenger explosives trace
detection equipment, procedures for evaluation and certification of the equipment, and
regulations for its use are all yet to be established. Regulations and screening procedures
have been established for explosives trace detection on luggage.9 Detection on passengers
is a more complicated venture, involving possible privacy concerns, greater difficulty in
sampling, and potentially different sensitivity requirements. Nevertheless, the current
luggage regulations could be a model for future certification criteria for passenger
screening. Procedures will also need to be established for the use of the equipment, such as how an operator should resolve detector alarms to distinguish genuine security threats
from false positives and innocuous true positives.
Impact on Screening Time
When multiplied by the large number of airline
passengers each day, even small increases in screening times may be logistically
prohibitive. The TSA goal for passenger wait time at airports is less than 10 minutes, and
screening systems reportedly operate at a rate between 7 to 10 passengers per minute;10
additional screening that slows passenger throughput and increases passenger wait time
may add to airport congestion and have a detrimental economic impact. A 1996 GAO
study stated that throughput goals for portal technologies at that time were equivalent to
6 passengers per minute.11 According to the same study, non-portal technologies, such
as secondary object analysis, had slightly higher throughput goals.
The TSA’s pilot deployment of passenger explosives trace detection equipment will
likely provide useful information on passenger throughput. If no appreciable increase in
screening times occurs, then passenger explosives screening may involve few additional
direct economic costs beyond those of procuring, deploying, operating, and maintaining
the equipment. If passenger throughput is drastically decreased, then alternatives for
passenger screening may need to be considered. In between these extremes, it may be
possible to moderate the economic impact by adding screening lanes or by using
explosives detection equipment only on those passengers who are selected for secondary
screening, as recommended by the 9/11 Commission as a possible initial step.
Erroneous and Innocuous Detection
A potential complication of explosives
trace detection is the accuracy of detector performance. False positives, false negatives,
and innocuous true positives are all challenges. If the detection system often detects the
presence of an explosive when there actually is none (a false positive) then there will be
a high burden in verifying results through additional procedures. Because of the large
volume of air passengers, even small false positive rates may be unacceptable.
Conversely, if the system fails to detect the presence of an explosive (a false negative)
then the potential consequences may be serious. Assuming the system has adequate
sensitivity to detect explosives traces in an operational environment, the detection
threshold or criteria required for an alarm can generally be adjusted, enabling a tradeoff
between false positives and false negatives, but neither can be eliminated entirely; the
appropriate balance may be a matter of debate.
Innocuous true positives occur when a passenger has been in contact with explosives,
but for legitimate reasons. Examples include individuals who take nitroglycerin for
medical purposes or individuals in the mining or construction industry who use explosives
in their work. Such passengers would be regularly subject to additional security scrutiny.
Similar issues arise from the current use of trace detection equipment on some airline
passenger carry-on baggage, and innocuous true positives in such cases are generally
handled without incident. The impact of innocuous true positives will likely depend on
their frequency and on the proportion of passengers subject to explosives trace detection.
Passenger Acceptance
Some passengers may have personal concerns about the
addition of passenger explosives trace detection to the screening process. Issues of
privacy may be raised by the connection between innocuous true positives and passenger
medical status or field of employment. Also, equipment that uses a vacuum “wand” or
puffs of air for sample collection may offend some passengers’ sense of propriety or
modesty. Passenger reluctance could then increase screening times. Allowing alternative
forms of screening, such as within privacy enclosures or through different imaging
technology, might mitigate passenger concerns in some cases.
Potential for Intentional Disruption
Another concern is the possibility that a passenger screening regimen that includes explosives trace detection could be exploited to intentionally disrupt the operation of an airport. The dissemination of trace quantities of an explosive material on commonly touched objects within the airport might lead to many positive detections on passengers. This would make trace detection less effective
or ineffective for security screening, and might disrupt airport operations generally until
alternative screening procedures, such as enhanced baggage screening by TSA personnel,
could be put in place or the contamination source could be identified and eliminated.
Research and Development
The DHS and its predecessor agencies have historically been the main funders of research on explosives detection for airport use. (Most of this research has focused on detecting explosives in baggage rather than on passengers.) Several other federal agencies, however, also fund research related to trace explosives detection. These include the Departments of Energy and Justice, the National
Institute for Standards and Technology, and the interagency Technical Support Working
Group. Much of this research has been dedicated to overcoming technical challenges,
such as increasing sensitivity and reducing the time required for sample analysis.
A different research challenge is the detection of novel explosives. Detectors are
generally designed to look for specific explosives, both to limit the number of false or
innocuous positives and to allow a determination of which explosive has been detected.
As a result, novel explosives are unlikely to be detected until identifying characteristics
and reference standards have been developed and incorporated into equipment designs.
Unlike imaging techniques for detecting bulk quantities of explosives, trace analysis
provides no opportunity for a human operator to identify a suspicious material based on
experience or intuition.
Liquid explosives are a novel threat that has been of particular interest since August
2006, when British police disrupted a plot to bomb aircraft using liquids. The DHS is
evaluating technologies to detect liquid explosives.12 Its efforts are mainly focused on
bulk detection, such as scanners to test the contents of bottles. Like solid explosives,
however, liquids might be found through trace detection, if the trace detection system is
designed to look for them.
the 9/11 Commission, recommended that Congress and the Transportation Security
Administration give priority attention to screening airline passengers for explosives.
The key issue for Congress is balancing the costs of mandating passenger explosives
detection against other aviation security needs. Passenger explosives screening
technologies have been under development for several years and are now being deployed
in selected airports. Their technical capabilities are not fully established, and
operational and policy issues have not yet been resolved. Critical factors for
implementation in airports include reliability, passenger throughput, and passenger
privacy concerns. Presuming the successful development and deployment of this
technology, certification standards, operational policy, and screening procedures for
federal use will need to be established. This topic continues to be of congressional
interest, particularly as the 110th Congress reexamines implementation of the 9/11
Commission’s recommendations via H.R. 1 and S. 4.
Introduction
In its discussion of strategies for aviation security, the 9/11 Commission
recommended that:
The TSA [Transportation Security Administration] and the Congress must give
priority attention to improving the ability of screening checkpoints to detect
explosives on passengers. As a start, each individual selected for special screening
should be screened for explosives.
The Intelligence Reform and Terrorism Prevention Act of 2004 (P.L. 108-458)
directed the Department of Homeland Security (DHS) to place high priority on developing
and deploying equipment for passenger explosives screening; required TSA, part of DHS,
to submit a strategic plan for deploying such equipment; and authorized additional
research funding. It also required that passengers who are selected for additional
screening be screened for explosives, as an interim measure until all passengers can be
screened for explosives. Congressional interest in this topic continues, particularly as the
110th Congress reexamines implementation of the 9/11 Commission’s recommendations.
The Implementing the 9/11 Commission Recommendations Act of 2007 (H.R. 1) would
require TSA to issue the strategic plan called for by P.L. 108-458 within seven days of
passage and would establish a Checkpoint Screening Security Fund, paid for with fees on
airline passengers, to develop and deploy equipment for explosives detection at screening
checkpoints. The Improving America’s Security Act of 2007 (S. 4) would require DHS
to issue the same strategic plan within 90 days of passage and begin its implementation
within one year of passage. The U.S. Troop Readiness, Veterans’ Health, and Iraq
Accountability Act, 2007 (H.R. 1591, the FY2007 supplemental appropriations bill)
would provide an additional $45 million for expansion of checkpoint explosives detection
pilot systems. This report discusses the current state of passenger explosives trace
detection and related policy issues.
Current State of Passenger Explosives Trace Detection
Explosives detection for aviation security has been an area of federal concern for
many years. Much effort has been focused on direct detection of explosive materials in
carry-on and checked luggage, but techniques have also been developed to detect and
identify residual traces that may indicate a passenger’s recent contact with explosive
materials. These techniques use separation and detection technologies, such as mass
spectrometry, gas chromatography, chemical luminescence, or ion mobility spectrometry,
to measure the chemical properties of vapor or particulate matter collected from
passengers or their carry-on luggage. Several technologies have been developed and
deployed on a test or pilot basis. Parallel efforts in explosives vapor detection have
employed specially trained animals, usually dogs.
The effectiveness of chemical trace analysis is highly dependent on three distinct
steps: (1) sample collection, (2) sample analysis, and (3) comparison of results with
known standards.2 If any of these steps is suboptimal, the test may fail to detect
explosives that are present. When trace analysis is used for passenger screening,
additional goals may include nonintrusive or minimally intrusive sample collection, fast
sample analysis and identification, and low cost. While no universal solution has yet been
achieved, ion mobility spectrometry is most often used in currently deployed equipment.
In 2004, TSA began pilot projects to deploy portal trace detection equipment for
operational testing and evaluation. In the portal approach, passengers pass through a
device like a large doorframe that can collect, analyze, and identify explosive residues on
the person’s body or clothing. The portal may rely on the passenger’s own body heat to
volatilize traces of explosive material for detection as a vapor, or it may use puffs of air
that can dislodge small particles as an aerosol. Portal deployment is ongoing.3
One alternative to portals is to collect the chemical sample using a handheld vacuum
“wand”. Another is to test an object handled by the passenger, such as a boarding pass,
for residues transferred from the passenger’s hands. In this case, the secondary object is
used as the carrier between the passenger and the analyzing equipment.4
The olfactory ability of dogs is sensitive enough to detect trace amounts of many
compounds, but several factors have inhibited the regular use of canines for passenger
screening. Dogs trained in explosives detection can generally only work for brief periods,
have significant upkeep costs, are unable to communicate the identity of the detected
explosives residue, and require a human handler when performing their detection role.5
In addition, direct contact between dogs and airline passengers raises liability concerns.
Detection of Bulk Explosives
Direct detection of explosives concealed on
passengers in bulk quantities has been another area of federal interest. Technology
development efforts in this area include portal systems based on techniques such as x-ray
backscatter imaging, millimeter wave energy analysis, and terahertz imaging.6 As such
systems detect only bulk quantities of explosives, they would not raise “nuisance alarms”
on passengers who have recently handled explosives for innocuous reasons. Some
versions could simultaneously detect other threats, such as nonmetallic weapons. On the
other hand, trace detection techniques are also likely to detect bulk quantities of
explosives and may alert screening personnel to security concerns about a passenger who
has had contact with explosives but is not actually carrying an explosive device when
screened. Current deployments for passenger screening are focused on trace detection,
and the remainder of this report does not discuss bulk detection. However, many of the
policy issues discussed below would apply similarly to bulk detection equipment.
Policy Issues
Any strategy for deploying and operating passenger explosives detection portals must
consider a number of challenges. Organizational challenges include deciding where and
how detectors are used, projecting costs, and developing technical and regulatory
standards. Operational challenges include maximizing passenger throughput, responding
to erroneous and innocuous detections, ensuring passenger acceptance of new procedures,
minimizing the potential for intentional disruption of the screening process, and providing
for research and development into future generations of detection equipment, including
techniques for detecting novel explosives. For security reasons, many technical details
of equipment performance are not publicly available, which makes independent analysis
of technical performance challenging.
Equipment Location and Use
An important component of a deployment
strategy is identifying where and how passenger explosives detection equipment will be
used. Portals could be deployed widely, so that all locations benefit from them, or they
could be used only at selected locations, where they can most effectively address and
mitigate risk. In any given location, portals could be used as a primary screening
technology for all passengers, or as a secondary screening technology for selected
passengers only. Widespread deployment and use for primary screening might provide
more uniform risk reduction, but would require many more portals and thus increase
costs.
Cost of Operation
The total cost of deploying explosives detection equipment
for passenger screening is unknown. According to TSA, the portal systems currently
being deployed in U.S. airports cost more than $160,000 each.7 Document scanning
systems are somewhat less expensive; according to a 2002 GAO study, similar tabletop
systems used for screening carry-on baggage can cost from $20,000 to $65,000.8 It is
possible that technology improvements or bulk purchasing could lower costs. The
number of devices required would depend on throughput rates, device reliability and
lifetime, and deployment strategy. The United States has more than 400 commercial
passenger airports; if equally distributed, several thousand devices might be required,
corresponding to a total capital cost for equipment of up to hundreds of millions of
dollars. Installation and maintenance costs would be additional. Operating the equipment
would require additional screening procedures and might lead to costs for additional
screening personnel, or else create indirect costs by increasing passenger wait times. It
is unknown whether the personnel limit for TSA screeners, currently set at 45,000 full
time equivalent screeners nationwide (P.L. 108-90), could accommodate the potential
additional staffing requirements.
Standards, Certification, Regulation, and the Establishment of Screening Procedures
Standards for the performance of passenger explosives trace
detection equipment, procedures for evaluation and certification of the equipment, and
regulations for its use are all yet to be established. Regulations and screening procedures
have been established for explosives trace detection on luggage.9 Detection on passengers
is a more complicated venture, involving possible privacy concerns, greater difficulty in
sampling, and potentially different sensitivity requirements. Nevertheless, the current
luggage regulations could be a model for future certification criteria for passenger
screening. Procedures will also need to be established for the use of the equipment, such as how an operator should resolve detector alarms to distinguish genuine security threats
from false positives and innocuous true positives.
Impact on Screening Time
When multiplied by the large number of airline
passengers each day, even small increases in screening times may be logistically
prohibitive. The TSA goal for passenger wait time at airports is less than 10 minutes, and
screening systems reportedly operate at a rate between 7 to 10 passengers per minute;10
additional screening that slows passenger throughput and increases passenger wait time
may add to airport congestion and have a detrimental economic impact. A 1996 GAO
study stated that throughput goals for portal technologies at that time were equivalent to
6 passengers per minute.11 According to the same study, non-portal technologies, such
as secondary object analysis, had slightly higher throughput goals.
The TSA’s pilot deployment of passenger explosives trace detection equipment will
likely provide useful information on passenger throughput. If no appreciable increase in
screening times occurs, then passenger explosives screening may involve few additional
direct economic costs beyond those of procuring, deploying, operating, and maintaining
the equipment. If passenger throughput is drastically decreased, then alternatives for
passenger screening may need to be considered. In between these extremes, it may be
possible to moderate the economic impact by adding screening lanes or by using
explosives detection equipment only on those passengers who are selected for secondary
screening, as recommended by the 9/11 Commission as a possible initial step.
Erroneous and Innocuous Detection
A potential complication of explosives
trace detection is the accuracy of detector performance. False positives, false negatives,
and innocuous true positives are all challenges. If the detection system often detects the
presence of an explosive when there actually is none (a false positive) then there will be
a high burden in verifying results through additional procedures. Because of the large
volume of air passengers, even small false positive rates may be unacceptable.
Conversely, if the system fails to detect the presence of an explosive (a false negative)
then the potential consequences may be serious. Assuming the system has adequate
sensitivity to detect explosives traces in an operational environment, the detection
threshold or criteria required for an alarm can generally be adjusted, enabling a tradeoff
between false positives and false negatives, but neither can be eliminated entirely; the
appropriate balance may be a matter of debate.
Innocuous true positives occur when a passenger has been in contact with explosives,
but for legitimate reasons. Examples include individuals who take nitroglycerin for
medical purposes or individuals in the mining or construction industry who use explosives
in their work. Such passengers would be regularly subject to additional security scrutiny.
Similar issues arise from the current use of trace detection equipment on some airline
passenger carry-on baggage, and innocuous true positives in such cases are generally
handled without incident. The impact of innocuous true positives will likely depend on
their frequency and on the proportion of passengers subject to explosives trace detection.
Passenger Acceptance
Some passengers may have personal concerns about the
addition of passenger explosives trace detection to the screening process. Issues of
privacy may be raised by the connection between innocuous true positives and passenger
medical status or field of employment. Also, equipment that uses a vacuum “wand” or
puffs of air for sample collection may offend some passengers’ sense of propriety or
modesty. Passenger reluctance could then increase screening times. Allowing alternative
forms of screening, such as within privacy enclosures or through different imaging
technology, might mitigate passenger concerns in some cases.
Potential for Intentional Disruption
Another concern is the possibility that a passenger screening regimen that includes explosives trace detection could be exploited to intentionally disrupt the operation of an airport. The dissemination of trace quantities of an explosive material on commonly touched objects within the airport might lead to many positive detections on passengers. This would make trace detection less effective
or ineffective for security screening, and might disrupt airport operations generally until
alternative screening procedures, such as enhanced baggage screening by TSA personnel,
could be put in place or the contamination source could be identified and eliminated.
Research and Development
The DHS and its predecessor agencies have historically been the main funders of research on explosives detection for airport use. (Most of this research has focused on detecting explosives in baggage rather than on passengers.) Several other federal agencies, however, also fund research related to trace explosives detection. These include the Departments of Energy and Justice, the National
Institute for Standards and Technology, and the interagency Technical Support Working
Group. Much of this research has been dedicated to overcoming technical challenges,
such as increasing sensitivity and reducing the time required for sample analysis.
A different research challenge is the detection of novel explosives. Detectors are
generally designed to look for specific explosives, both to limit the number of false or
innocuous positives and to allow a determination of which explosive has been detected.
As a result, novel explosives are unlikely to be detected until identifying characteristics
and reference standards have been developed and incorporated into equipment designs.
Unlike imaging techniques for detecting bulk quantities of explosives, trace analysis
provides no opportunity for a human operator to identify a suspicious material based on
experience or intuition.
Liquid explosives are a novel threat that has been of particular interest since August
2006, when British police disrupted a plot to bomb aircraft using liquids. The DHS is
evaluating technologies to detect liquid explosives.12 Its efforts are mainly focused on
bulk detection, such as scanners to test the contents of bottles. Like solid explosives,
however, liquids might be found through trace detection, if the trace detection system is
designed to look for them.
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