An explosion occurs as the result of a chemical reaction which releases a vast amount of physical or chemical energy accompanied by light, heat and sound in a short space of time. The cause of an explosion could range from a simple accident in the home to a major terrorist incident. Either way, an intricate investigation must take place to establish the exact circumstances of the event.
The construction of a bomb can be fairly simple, some requiring just a timer or remote control to create an initial spark, which ignites the bomb and triggers a blast. The recipes for making such a bomb are readily available online. The strength of a bomb can vary greatly, some yielding little power and others causing mass destruction, depending on the chemical reactants used and the construction of the device itself.
Detonating cords are common parts of many explosive devices. These are plastic tubes filled with powdered explosive, for instance PETN (pentaerythritol tetranitrate), commonly used to link charges through the transmission of the detonation shock wave or as the explosive charge themselves. Explosive devices will often contain boosters, components which amplify and transmit the shockwave between the detonator and main charge. Many bombs require some kind of trigger to initiate the detonation, so timers or remote control devices are commonly used. These start the initial charge, which then goes on to ignite the explosive and detonate the bomb.
The site of an explosion must first be deemed safe before officers may proceed with their investigation. As with in fire investigations, consideration must be given to the structural integrity of buildings, dangerous materials such as glass and metal, and flammable or toxic substances. In bombing cases there is always the possibility that second devices have been placed, either bombs that did not detonate as planned or those specifically designed to harm people responding to the incident. Therefore the area should be deemed structurally safe, experts should be called in to locate any further explosives, and appropriate protective clothing should be worn. As with any crime scene, a cordon will be placed around the area to preserve evidence and control who can enter the scene. The diameter of the cordon will be dependent on the incident, as an unexploded device requires a smaller cordon as no debris is involved.
Initially the investigation will establish whether an explosion has actually occurred. Explosions will result in specific damage being caused to surrounding areas, the presence of which may indicate such an incident has taken place. Therefore investigators will search for this characteristic damage. Textiles will take on specific material when subjected to the heat caused by an explosion, with many melting and once again solidifying, displaying clubbed damage. Surfaces slightly further away, such as rooftops and stationary vehicles, are likely to collect soot deposits from materials burned during the explosion. The pressure and heat of the blast will cause specific damage to nearby surfaces, the kind of damage being specific to the material. Once confirmed, the origin of the explosion will be located. The presence and depth of specific damage or a crater may indicate the origin, or seat, of the explosion. It must then be established whether the incident was caused by maliciously placed explosives or if it was an accident, such as a gas leak. Investigators will be on the lookout for both signs of potential gas leaks and similar but also for indicators of bomb use.
Before and throughout the investigation any witnesses to the incident will be interviewed. Witness reports are useful for gathering information on any occurrences leading up to the explosion, details of the explosion itself, and anything else that may be relevant, such as the sightings of suspicious individuals.
During the investigation of an explosion scene, investigators must search far and wide for possible fragments, as much of the bomb is thrown far from the epicentre due to the heat and kinetic energy produced. Fragments may be found on rooftops, under other debris, and even embedded in other objects or victims. All fragments should be collected to ensure the bomb can be reconstructed. Such fragments may also carry fingerprints belonging to the perpetrator. Components searched for at a crime scene include detonators, tapes, wires, timers, switches, and batteries. Once all present components have been collected, experts may be able to establish what kind of explosive device was used, including its construction and how it was triggered. The type of explosive used, its construction, or specific components used may be useful in locating the bomb maker, particularly if a series of explosions have occurred over a period of time. Similarly, particular terrorist organisations may be known for using a specific type of explosive. An investigation may often be conducted into recent purchases of certain substances. Explosives are generally strictly regulated by the government, and so a flag may be raised when an individual purchases a particular chemical. However materials used in bombs are often stolen or smuggled into the country, so the components may be more difficult to trace. Furthermore, bombs can be constructed from everyday products, thus tracing the purchase of such materials to a specific individual is not a viable option.
Investigations are not limited to the scene of the explosion incident itself, but can be extended to the homes of suspects. Homes may be examined and analysed for any indication of bomb construction, including bomb fragments and explosive substances. Surfaces will be swabbed to detect the presence of minute traces of explosive residues, either using chemical indicators or to collect samples for further laboratory analysis.
Types of Explosive
Explosions may have a chemical and physical element. The first relates to the chemical energy stored in the explosive molecules which produce the heat, light, sound, and kinetic energy. A physical explosion may be the sudden release of pressure, such as in the explosion of a container of gas.
A detonation is a subsonic explosion with a speed greater than the speed of sound in that material, with a pressure wave of up to 8500 metres per second. This occurs in high explosives. Detonations tend to have a shattering effect, and a particularly condensed explosion will cause a huge crater.
Deflagration is a type of subsonic combustion which occurs slower than the speed of sound in that material, creating a pressure wave of up to 1000 metres per second or less. It is usually propagated by thermal conductivity and is common to most every day fires. Condensed explosives undergoing deflagration produce extreme heat which will melt surrounding objects.
There are two kinds of explosive: low explosive and high explosive. Low explosives are characterised by their ability to burn in the same way any other combustible material would. They exert a throwing or pushing effect when the explosion occurs, with a blast front velocity below 400ms-1. Examples of low explosives, also known as propellants, include gunpowder and nitrocellulose. High explosives are much more dangerous, having more of a shattering effect and exploding at a rate of between 100 and 8500 metres per second. They have a blast front velocity of 400ms-1, and can reach up to 19000ms-1. High explosives can be primary or secondary. Primary high explosives are very sensitive, with an example being nitroglycerine. Secondary explosives are conversely fairly insensitive, with examples including PETN and TNT.
A variety of explosives are commonly encountered, their construction and mechanisms specific to their use.
Pipe Bombs – Simple construction generally consisting of a short length of pipe with both ends capped. One end is drilled and the detonator inserted, and either high or low explosives may be used. Parts of this bomb usually survive the blast and may hold evidence such as fingerprints and manufacturer details.
Vehicle Bombs – These generally use large quantities of low explosive with a high explosive booster. The bombs are placed inside vehicles which can generally be subtly left in the target location where they will often cause great destruction. It is usually possible to at least identify the make and model of the vehicle used and potentially even its owner.
Letter & Parcel Bombs – These generally contain a small amount of high explosive packaged inside an inconspicuous letter or parcel. Such bombs often yield forensic evidence as the devices rarely cause fires and so less evidence is destroyed.
Mines – These are concealed devices containing high explosive, commonly used by the military. Mines are often hidden and detonated when touched. Much forensic evidence can often be obtained from the device.
Suicide Bombers – Suicide bombers, usually an individual wearing a vest of explosives or carrying a device, are primarily utilised by terrorist organisations. These types of bombs allow for the explosives to literally be walked into the target location and placed close enough so that maximum destruction can be caused. The identity of bombers can often be ascertained from CCTV footage and witness statements. Parts of the bomber may even be found, as the upper torso and head sometimes survive the blast and are found a distance away from the epicentre of the blast. Many terrorist organisations will publish or advertise the identity of the bomber after the event has occurred.
Throughout the investigation various techniques may be utilised to detect explosive residues, either traces left behind by the explosive device or materials used in the production of the explosives.
The use of specially trained dogs is perhaps one of the oldest methods of detecting explosives. These canines are trained to use their exceptional sense of smell to detect and locate the presence of even the slightest traces of explosive material. They may equally be utilised at the crime scene itself to detect more explosive devices or residues, and in homes of suspects to establish whether the location bears any trace of such substances. The dogs may also be instructed to detect explosive scents on people and their clothing. You can read more about these trained dogs on the detection dogs page.
Similar to trained canines are electronic ‘sniffer’ devices. Explosives generally emit certain volatile components which the device can detect. The air is drawn through a filter, causing the components to collect where they can then be analysed, giving some indication as to its composition. However some explosives, such as plastic explosives, are not easily detected by these devices, therefore canines may be more useful in this respect. Detection techniques should always be used in unison rather than exclusively.
Away from the crime scene, standard analytical techniques may be used to detect and identify explosive substances. Mass spectrometry and gas chromatography are frequently used in the analysis of samples collected from the scene to identify any explosive residues.