Bomb (Explosion) Investigations

The investigation of explosions has a long history in forensic science and covers incidents ranging from accidents in the home or workplace to major terrorist attacks. An explosion is a sudden release of physical or chemical energy, carried on a high-pressure wave, and generally accompanied by an emission of heat, light, and sound. An explosion may result from a criminal act involving a bomb, but it can also occur by accident—if a spark ignites a leak of domestic gas, for example. Establishing the nature of an explosion can be a significant challenge to the forensic investigator. The high-pressure wave of an explosion can be extremely destructive, both to bystanders and any objects of materials in the vicinity. Analyzing this kind of evidence can be a very difficult task, particularly as an explosion is often followed by a fire. This causes complications for the investigators as much valuable evidence is then destroyed. There are also potential hazards to the investigators themselves in investigating a bomb site. There may be structures in danger of imminent collapse as well as exposure to dangerous materials such as broken glass , flammable or toxic vapors, or asbestos. In the case of a bombing, there is always a possibility that a second device has been placed to kill or maim those who respond to the explosion.

Explosions are an example of the Law of Conservation of Energy, which states that energy can neither be created nor destroyed, but it can be converted from one form into another. The two main classes of explosions are chemical and physical. When a bomb explodes, for instance, the chemical energy stored in the molecules of the explosive making up the device is converted into kinetic (movement) energy, heat, light, and sound. This is an example of a chemical explosion—probably the class of most interest to the forensic scientist. Physical explosions are the other main class requiring investigation. A typical example of a physical explosion is the sudden release of gas from a container of pressurized gas or liquid if it becomes overheated.

An explosive is a substance that can produce an explosion through a chemical reaction. When it is used illegally and to cause harm it is generally known as a bomb. Legitimate explosives include fireworks and blasting materials used in quarrying. Explosives generally contain fuel and an oxidant and it is the chemical reaction between them which releases stored chemical energy.

There are two types of explosions due to chemical reactions, which is reflected in the type of damage they cause at the scene. In a detonation, the speed at which the chemical reaction moves though the explosive is greater than the speed of sound in that material. The resulting pressure wave may move at up to 8,500 meters per second (9,296 yards per second). High explosives, such as dynamite, generally undergo detonation and have a characteristic shattering effect on their surroundings. A deflagration occurs when the speed of the chemical reaction of the explosion travels through the explosive slower than the speed of sound in the material. This creates a pressure wave moving at 1,000 meters (1094 yards) per second or less. The impact of low explosives, such as a mixture of air and gasoline vapor or sugar and potassium chlorate, is best described as pushing, rather than shattering, although they can still produce an enormous amount of damage. Depending on their nature, explosives may or may not need an initiating material, called a detonator, to set them off. A useful distinction can also be made between condensed explosives, which are solid or liquid, and dispersed explosives, consisting of aerosol or gas.

Investigation of the scene of an explosion aims to discover whether an explosion actually took place and, if so, whether it was an accident or a bomb. The forensic scientist will then try to find out what kind of explosion occurred, the materials involved and, in the case of a criminal act, they will work with the police to find out who was responsible. Examination of the scene and witness reports can establish whether an explosion has happened. Loud bangs, flashes, violent eruption of debris, shattering of nearby objects and formation of a crater where the event occurred are all indicative of an explosion. The investigator will look for evidence of a possible accident, such as a gas leak or creation of a cloud of flammable gas at the scene. If it looks as if a bomb caused the explosion, then the explosive device must found. This involves searching for the device itself and any detonator fragments which may be scattered among the debris. There may have been a timing device to allow the bomber time to get away, which would consist of electronic circuitry, wires, and batteries. The remains of the device will probably contain some residue from the explosive and may even bear fingerprints from the perpetrators. The construction of the device and how it was triggered may also be deduced from examination of these fragments.

The investigator will probably have to search far and wide at the scene of the explosion to recover bomb fragments. Some may be embedded in the bodies of victims, and here medical staff will need to carry out x rays to identify any evidence and, if possible, recover it for forensic investigation. A suicide bomber is, of course, an important source of such evidence. Suspect surfaces must be swabbed with various solvents to extract invisible chemical traces of explosive residue. There is nearly always a part of the bomb that did not explode and these residues can be very informative. Small items that may bear explosive residue can be placed in a beaker and agitated with a suitable solvent. The solvent has to be chosen to match the explosive—diethyl ether may be used for organic materials, while water dissolves inorganic materials such as potassium chlorate.

Once the samples are back in the laboratory, there are many sensitive analytical techniques, such as high performance liquid chromatography and thin layer chromatography , that can be used to assess the chemical nature of explosives and identify the trace evidence , comparing it with reference samples of explosives. Similar techniques can be used to sample for traces of explosives on suspects' hands and clothing. Comparison may be sufficient to link a suspect with a crime scene. The analysis of traces of explosives has to be done with great care and expertise because there is ample opportunity for cross contamination to occur. This means taking scrupulous care with the collection of the trace evidence and then using control samples throughout the analysis. If the explosive can be identified, the police investigation will look for buyers and sellers of that particular material.

Explosions often cause characteristic damage to nearby surfaces through a combination of the high temperature generated and the high pressure wave. A mottled irregular appearance, known as gas wash, results from a combination of melting and erosion of the surface material. Textiles may undergo characteristic clubbing damage as the polymer melts and then re-solidifies. On metal surfaces, microcraters may be visible on microscopic examination. Soot deposits on more distant surfaces, such as window frames, are also characteristic of an explosion.

The pattern of damage at the scene of an explosion will help the forensic scientist to determine what happened. The location and depth of any crater or the nature of structural damage such as broken windows can all help to locate the actual seat of explosion, for instance. The scene can also be very informative about the nature of the explosion too, as different combinations of explosive and explosion can give rise to characteristic types of damage. Detonation of a condensed explosive tends to produce a huge crater and very severe damage that involves pulverizing and shattering of nearby objects, even if they are made of tough materials like steel. A deflagration in a condensed explosive produces intense heat and could bend or melt objects rather than cutting them. Detonation is rare with dispersed explosives, but deflagration gives a pattern in which most of the damage may occur some way from the explosion itself owing to a pushing out effect. In one example, a natural gas explosion caused only superficial burns to two people in the basement underneath the room where it occurred, yet the incident was violent enough to blow furniture out of the building.

Some explosions are of a mixed type. Petrol (gasoline) bombs are often used by terrorists and typically involve using a small charge of high explosive to disperse and ignite petrol, which is a flammable liquid. This event involves detonation of a high explosive and deflagration of a dispersed explosive. The detonation will produce damage close to the point where the bomb was set off, while the deflagration will produce damage further away.

see also Accelerant; Bomb damage, forensic assessment; Bomb detection devices.