Investigation of bombings

Definition: Incidents involving weapons that explode and release destructive shock waves and shrapnel that damage buildings and other property and injure and kill people.

Significance: In modern society, law-enforcement agencies are increasingly faced with the investigation of crimes involving explosives. Forensic scientists and law-enforcement officers use a number of different techniques to detect explosives before they can be used in bombs, and, after bombings have taken place, they examine the resulting debris for evidence that can link the explosions to the perpetrators.

Bombs create destructive shock waves, flying shrapnel, and intense heat and flame capable of destroying objects and killing people. As the materials needed for bomb making (especially such dual-use products as fertilizer) and the technical information needed to construct and detonate bombs have become more readily available than in the past, in large part because of the advent of the Internet, bombings have increased in frequency. The availability of potentially explosive chemicals, dynamite, and, in some countries, military explosives has provided the criminally disposed with the ability to wield very destructive and deadly weapons. Furthermore, the news coverage that inevitably follows bombing incidents may inadvertently embolden those so inclined to carry out additional bombing attacks.

Types of Explosives

Every bomb has an igniter, a primer or detonator, and a main charge. Most kinds of bombs are confined within some sort of shell, such as a pipe or a box. The igniter may be either a fuse or a primer. A primer is a small explosive charge that may be ignited by flame, electrical spark, or friction. When the primer is ignited, it explodes, causing the bomb’s main charge to detonate. A firearm cartridge, for example, is an explosive that is set off when a shock-sensitive primer located at one end of the cartridge is struck by a pin. When a gun’s firing pin strikes the primer on a firearm cartridge, the primer explodes and ignites the main charge, the smokeless black powder located behind the bullet. The explosion of the main charge is what forces the bullet to travel through the barrel of the gun and downrange.

The speed at which an explosive detonates determines whether it is classified as a low or high explosive. Low explosives, such as black and smokeless powder, are typically used as propellants for ammunition and rockets because they burn relatively slowly. Most homemade bombs tend to be low explosives because they are often constructed with black powder, which is easy to obtain from gun stores. High explosives such as dynamite and C-4, in contrast, produce more of a smashing, shattering effect.

High explosives may be divided into two types, primary and secondary. Primary high explosives tend to be sensitive to heat, shock, and friction and detonate very easily. Because of this, they are generally used in primer devices to set off larger, secondary explosives. Secondary high explosives tend to be relatively insensitive to heat, shock, and friction and usually require a primary charge explosion to detonate. In most cases, this involves the use of a blasting cap, initiated by a burning fuse or by an electrical current. Homemade bombs are usually initiated by an electronic blasting cap wired to a battery that is switched on by a device such as a clock, mercury switch, vehicle ignition switch, or cell phone ring.

Dynamite is a high explosive known for producing a quick shattering effect. It is mainly used for construction, mining, and demolition. When it was first developed in 1866, it was made from nitroglycerin, diatomaceous earth (soft, chalklike sedimentary rock), and sodium carbonate wrapped in distinctive red paper. The “kick” that is produced by dynamite is derived from the nitroglycerin it contains. Nitroglycerin is a very powerful shock-sensitive explosive, meaning that vibrations may cause it to explode. This makes it very dangerous to handle. However, when diatomaceous earth and sodium carbonate are combined with nitroglycerin to create dynamite, the nitroglycerin becomes more stable and safe to handle.

The explosive strength of a stick of dynamite is designated by the percentage of nitroglycerin it contains; for example, in a 60 percent grade stick of dynamite, 60 percent of the stick consists of nitroglycerin. The actual blasting power of a stick of dynamite, however, is not in proportion to its grade percentage markings. That is, a 60 percent grade stick of dynamite is not three times as powerful as a 20 percent grade stick; it is only about one and one-half times as strong.

By the early years of the twenty-first century, the use of nitroglycerin-based dynamite had all but disappeared. Dynamite was replaced by ammonium nitrate-based explosives, which are more stable and useful in wet conditions. Ammonium nitrate/fuel oil (ANFO) explosives are high explosives that are often used in the mining and construction industries. They consist of ammonium nitrate soaked in fuel oil and require a primer explosive to detonate. About 80 percent of the explosives used in North America are ANFO explosives. The availability of ammonium nitrate in the form of fertilizer makes it a readily obtainable ingredient for homemade explosives, and its use in bombs has become a trademark of various criminal and terrorist groups around the globe. Timothy McVeigh used a variation of an ANFO bomb when he attacked the Alfred P. Murrah Federal Building in Oklahoma City in 1995, killing 168 people and injuring hundreds more.

The explosive known as RDX is currently the high explosive most commonly used by the U.S. military. It is second in strength only to nitroglycerin. RDX is widely used in plastic explosives, detonators, artillery rounds, Claymore mines, and demolition kits. It is combined with plasticizers to make C-4, which is a pliable, puttylike explosive that can be molded into a variety of shapes and has a long shelf life. It is believed that al-Qaeda used C-4 in 1996 to blow up the Khobar Towers (a military housing complex) in Saudi Arabia and again in 2000 in its attack on the military destroyer the USS Cole. In the Khobar Towers bombing, nineteen U.S. service personnel were killed. In the bombing of the Cole, seventeen sailors were killed and thirty-nine others were injured.

Triacetone triperoxide (TATP) is an explosive created through the combination of the common ingredients of acetone and hydrogen peroxide with a catalyst such as hydrochloric acid. Persons who are so inclined can purchase its base ingredients (drain cleaner, bleach, and acetone) easily and without attracting suspicion, and instructions for making TATP can be found on the Internet. In its finished form, this explosive is almost undetectable by substance-detection dogs or by conventional bomb-detection systems. Because of this, the Palestinian militant organization Hamas has favored TATP for use by suicidebombers sent into Israel. Al-Qaeda has also used it when conducting terror missions abroad. TATP was included as a trigger in the shoe bomb that Richard Reid intended to detonate on a flight from Paris, France, to Miami, Florida, in 2001. It is also the type of explosive that was used in the 2005 public transit bombings in London, England, which killed fifty-two commuters and injured seven hundred people. The drawback of TATP, from a criminal or terrorist’s point of view, is that it is highly unstable and sensitive to heat and friction.

Detecting Explosives at Airports

Terrorists around the globe have successfully used explosives to end lives and undermine public confidence in air travel. In response to such threats, airports and their cargo terminals employ a number of different techniques and technologies to aid in the identification and interdiction of explosives.

X-ray machines are used to scan large numbers of people and items to identify hidden suspicious shapes that could indicate the presence of bombs. Because it is possible that such explosive devices could be hidden inside electronic equipment, such as laptop computers, security measures often include chemical analyses. In such a test, a swab is wiped across a piece of electronic equipment, such as a laptop, and is then placed into a device that heats it up and performs a spectrographic analysis of the resulting vapors. The machine searches for traces of nitrogen, which is found in the majority of explosives.

Trace-detection machines (sniffers), which look like metal detectors, search for explosives by blowing air over persons or their luggage. The blowing air releases particles from the surface of the person or object of interest, and the machine then processes the air and analyzes it for traces of known explosives. Airport security measures also include the use of dogs that have been trained to alert their handlers by sitting near any objects or persons that give off the telltale odors of explosives.

Responding to Bomb Threats

Most bomb threats turn out to be nothing more than prank phone calls from misguided individuals who take pleasure in causing others fear and inconvenience. Unfortunately, those whose true intent is to kill, maim, and destroy are unlikely to notify their intended victims prior to the detonation of their bombs. When a bombing does occur, individuals who have specialized training in bomb disposal, bomb-site investigation, forensic analysis, and criminal investigation work together to determine what happened so that those who are responsible may be apprehended.

When a bomb threat is called in, the authorities who are given the task of responding to the scene (the first responders) need to enlist the assistance of people who are familiar with the area, such as building managers and employees, because such persons may be more adept at determining whether something is out of place than someone who is not as familiar with the surroundings. Those participating in a search for a bomb must turn off all their radios and transmitters before they begin the search, because the signals these devices emit may set off an explosion. When searchers first enter a room in a location where a bomb may have been planted, they pay special attention to items such as unattended bags, boxes, baby carriers, briefcases, trash cans, flowerpots, incoming mail, and panels in the ceiling that may be easily pushed up. Experts also recommend that when searchers enter a room, they should stand quietly in the room’s center, close their eyes for several seconds, and listen. Unusual noises may indicate the location of a bomb.

If a bomb is found, the searchers are careful not to touch it, because contact may cause it to explode. Only bomb disposal personnel are tasked with handling any suspected devices that are located. Bomb squads in larger police departments use robots to approach and detonate certain bombs. After a bomb is found, the area is cleared and the crime scene is secured to prevent further contamination. Emergency services are requested from bomb technicians, firefighters, emergency medical personnel, and law-enforcement officers, and a search for secondary explosive devices is then conducted.

Investigating Bomb Explosions

When an explosion occurs, law-enforcement personnel must identify scene hazards such as the possibility of building collapse, hazardous chemicals, and secondary explosives. Bombing scenes may contain secondary explosive devices specifically designed to kill or maim public safety responders. If a suspected secondary device is located, the area must be evacuated immediately and bomb disposal personnel must be contacted. As soon as conditions permit, investigators need to establish a security perimeter that restricts access into and out of the scene; they also begin documenting the scene (taking notes, identifying witnesses, and videotaping bystanders).

During an initial scene walk-through, investigators pay special attention to various safety concerns, such as structural damage, the possibility of the presence of secondary devices and unconsumed explosive materials, failed utilities, and hazardous materials. Following this walk-through, the investigators meet with available emergency responders and investigative personnel to determine what resources, equipment, and additional personnel may be needed.

The search for evidence typically starts at the seat of the blast, which is usually indicated by a crater, and spirals out in ever-increasing circles. The scene is documented with both written and photographic records before anything is removed or disturbed. The material at the scene is then sorted in an attempt to recover the materials that were used to construct the bomb. All of the personnel involved in the search must wear disposable gloves, shoe covers, and overalls so that they do not contaminate evidence and compromise the investigation.

To uncover clues to the construction and thus the origin of a bomb, investigators usually sift material from the blast scene through a series of increasingly finer mesh screens to collect portions of the explosive device for analysis. For instance, if a pipe bomb was used, a forensic investigator may find the bomb’s end cap; in many cases, this part of a pipe bomb will retain small specks of unexploded material that becomes trapped in the threading. These small specks of explosive may then be used to trace the origins of the materials used to construct the bomb. Investigative leads may also develop from tool marks left on a pipe from a vise used in cutting and threading. Other clues that may aid an investigation include the type of wire that was used, the type of timing device used, the particular type of wrapper paper (indicating the origin of a piece of dynamite), or a unique method of bomb construction.

The materials from the scene that are collected for laboratory examination are placed in sealed containers and labeled. Soil and other soft materials are placed in metal containers or plastic bags. Evidence samples that are packaged in plastic bags must not be kept next to each other, because it has been demonstrated that some explosives can diffuse through plastic and contaminate nearby containers.

Bombing victims should also be examined for evidence, as bomb component fragments may be found on or in their clothing or bodies. Autopsies should include full-body X rays.

When the debris evidence from a bomb scene arrives at the laboratory, it is examined microscopically, and an acetone wash is often used to extract explosives from the debris. Chromatographic techniques (which can separate and identify the components in chemical mixtures) may then be used to determine the types of explosives that were used.

Explosive residues are often collected at bomb scenes with a portable machine called an ion mobility spectrometer (IMS). The IMS uses a vacuum to suck in explosive residues from surfaces. Depending on the types of surfaces found at a bomb scene, however, investigators may collect explosive residues more efficiently by wiping the surfaces down with paper disks and then using the IMS to collect the residues off the disks. Once the residues are in the IMS, they are vaporized into electronically charged molecules or ions. Identification of the size and structure of the molecules and ions enables investigators to determine the types of explosives that were detonated at the bomb scene.

Investigators often examine bomb blast craters using an ultraviolet light and magnetic probe in the hope of finding small particles, called taggants, that are sometimes put into explosives by manufacturers. Taggants are tiny color-coded magnetic fluorescent chips the size of sand grains. The color of the fluorescent chips indicates where an explosive was made and when it was produced. Switzerland requires all explosives manufacturers in that nation to add taggants to their products. The U.S. government has not taken such a step, but increasing concerns about terrorism may eventually result in a similar requirement for American manufacturers.

Bibliography

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Gaensslen, R. E., Howard A. Harris, and Henry C. Lee. Introduction to Forensic Science and Criminalistics. New York: McGraw-Hill, 2008. Addresses the types of forensic science techniques used in crime laboratories in criminal cases and by private examiners in civil cases. Discusses various crime scene procedures and analyses, physical pattern evidence, biological evidence, and chemical and materials evidence.

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National Institute of Justice. A Guide for Explosion and Bombing Scene Investigation. Washington, D.C.: Author, 2000. Outlines the tasks that investigators should consider at every explosion scene and provides guidance on the procurement of equipment and tools, prioritization of initial response efforts, evaluation of the scene, documentation of the scene, and processing of evidence at the scene.

Saferstein, Richard. Criminalistics: An Introduction to Forensic Science. 9th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2007. Comprehensive introductory textbook addresses the role of science in the criminal justice system. Includes in-depth discussion of the technologies that law-enforcement agencies use to apprehend criminals and the use of trace evidence to link perpetrators to crime scenes.

Simonsen, Clifford E., and Jeremy R. Spindlove. Terrorism Today: The Past, the Players, the Future. 3d ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2007. Provides background on the history and legal issues associated with terrorism and discusses the types of terrorism and terrorist groups found around the world. Includes suggestions regarding counterterrorism tactics and speculation on the directions terrorism may take in the future.

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