No abstract

Six test firings of large improvised explosive devices were carried out. The principal objectives of the firings were to measure the physical effects of the explosions upon representative objects placed nearby and to recover any chemical traces deposited on these objects. The results are intended for use as an aid in determining the approximate size and type of an explosive employed in terrorist attacks. Three 454 kg charges of a mixture of calcium ammonium nitrate (CAN) fertilizer and sugar, and three 2268 kg charges of a similar mixture, all confined in cylindrical steel containers were fired. Each charge was surrounded by 19 road signs mounted on posts and four vehicles, to act as witness materials. The analysis of aqueous swab extracts taken from the witness materials after firing showed the recovery of nitrate, ammonium and low levels of glucose. No sucrose was detected. Nitrate was usually recovered in greater quantities than ammonium and recovery generally decreased with increasing distance from the charges in any given direction. Quantities recovered from objects placed at the same distance in different directions varied considerably. Patterns of physical damage to the witness materials could be discerned according to their distance from the charge and the size of the charge. The velocities of detonation and air blast effects were measured.

Recent criminal acts in the United Kingdom, United States and other countries have demonstrated the dangers to public safety from the criminal use of improvised explosives on a large scale. Four sets of trials were carried out over four years, partly in collaboration with the United States Federal Bureau of Investigation, involving the firing of large bombs, mostly fertilizer based. The principal objectives of the firings were to measure the physical effects of the explosions upon objects representative of those that would be found at a real bomb scene and to recover any chemical traces deposited on these objects. The results are intended for use as an aid in determining the approximate size and type of an explosive employed in a terrorist attack. This paper describes the background behind the trials, the procedures for preparation of witness materials and charges, and the collection and analysis of physical and chemical evidence.

The collection of both physical and chemical evidence from the scene of a large bomb can be useful in determining the type of explosive charge used and also its approximate size. A shortage of practical experience of such explosives on a large scale can hinder the collection and interpretation of such evidence. Six charges of calcium carbonate (limestone) ammonium nitrate/sugar (LAN/S) improvised explosive devices were fired (three at 454 kg and three at 2268 kg) together with one charge of 2,4,6-trinitrotoluene (TNT, 454 kg) and one charge of ammonium nitrate fuel oil (ANFO, 2268 kg). The charges were surrounded by vehicles, roadsigns, and lampposts that acted as witness material to provide both physical and chemical evidence. Analyses showed that ammonium, higher levels of nitrate, and some sugars were recovered from the LAN/S firings; low levels of nitrate from the ANFO firing; significant levels of TNT from the TNT firing. Levels of recovery generally decreased with increasing distance from the charge. The pattern of physical damage to the witness pieces at given distances from the charge was recorded. The velocities of detonations were measured and the corresponding TNT equivalence calculated.

The forensic scientist investigating the scene of an explosion must start with the determination of whether a bomb or some mishap caused the event. Locating pieces of the explosive device or explosive residue aids in fixing the event as a bombing. The investigator is advised that proper evidence collection requires preparation before the bombing as to how to locate the most useful evidence, how to avoid contamination, how to sift explosion debris, how to conduct the chemical analyses, and what questions he must answer in a court of law. Examples from actual bombings are given and a compendium of literature references is offered.