Types and Characteristics of Explosives and IEDs

Countering the explosives threat is a difficult challenge as there are many types of explosives and different forms of bombs. The many different types of explosives are loosely categorized as military, commercial, and a third category called homemade explosives (HME) because they can be constructed with unsophisticated techniques from everyday materials. The military explosives include, among others, the high explosives PETN and RDX, and the plastic explosives C-4 and Semtex. The military uses these materials for a variety of purposes, such as the explosive component of land mines, shells, or warheads. They also have commercial uses such as for demolition, oil well perforation, and as the explosive filler of detonation cords. Military explosives can only be purchased domestically by legitimate buyers through explosives distributors and typically terrorists have to resort to stealing or smuggling to acquire them. RDX was used in the Mumbai passenger rail bombings of July 2006. PETN was used by Richard Reid, the "shoe bomber" in his 2001 attempt to blow up an aircraft over the Atlantic Ocean as well as a component involved in the attempted bombing incident on board the Northwest Airline Flight 253 on Christmas Day 2009.

Commercial explosives, with the exception of black and smokeless powders, also can only be purchased domestically by legitimate buyers through explosives distributors. These are often used in construction or mining activities and include, among others, trinitrotoluene (TNT), ammonium nitrate and aluminum powder, ammonium nitrate and fuel oil (ANFO), black powder, dynamite, nitroglycerin, smokeless powder, and urea nitrate. Dynamite was likely used in the 2004 Madrid train station bombings, as well as the Sandy Springs, Georgia abortion clinic bombing in January, 1997. ANFO was the explosive used in the Oklahoma City, Oklahoma bombings in 1995.

The common commercial and military explosives contain various forms of nitrogen. The presence of nitrogen is often exploited by detection technologies some of which look specifically for nitrogen (nitro or nitrate groups) in determining if a threat object is an explosive. HMEs, on the other hand, can be created using household equipment and ingredients readily available at common stores and do not necessarily contain the familiar components of conventional explosives. On February 22, 2010, Najibullah Zazi pleaded guilty to, among other things, planning to use TATP26 to attack the New York City subway system. Also, HMEs using TATP and concentrated hydrogen peroxide, for example, were used in the July 2005 London railway bombing. TATP can be synthesized from hydrogen peroxide, a strong acid such as sulfuric acid, and acetone, a chemical available in hardware stores and found in nail polish remover, and HMTD27 can be synthesized from hydrogen peroxide, a weak acid such as citric acid, and hexamine solid fuel tablets such as those used to fuel some types of camp stoves and that can be purchased in many outdoor recreational stores. ANFO is sometimes misrepresented as a homemade explosive since both of its constituent parts-ammonium nitrate, a fertilizer, and fuel oil-are commonly available.

When used, for example, in terrorist bombings, explosives are only one component of an IED. Explosive systems are typically composed of a control system, a detonator, a booster, and a main charge. The control system is usually more mechanical or electrical in nature. The detonator usually contains a small quantity of a primary or extremely sensitive explosive. The booster and main charges are usually secondary explosives which will not detonate without a strong shock, for example from a detonator. IEDs will also have some type of packaging or, in the case of suicide bombers, some type of harness or belt to attach the IED to the body. Often, an IED will also contain packs of metal-such as nails, bolts, or screws-or nonmetallic material which are intended to act as shrapnel or fragmentation, increasing the IED's lethality. The various components of an IED-and not just the explosive itself-can also be the object of detection.

The initiation hardware, which may be composed of wires, switches, and batteries, sets off the primary charge in the detonator which, in turn, provides the shock necessary to detonate the main charge. The primary charge and the main charge are often different types and categories of explosives. For example, in the attempted shoe bombing incident in 2001, the detonator was a common fuse and paper-wrapped TATP, while PETN was the main charge. While in the past the initiation hardware of many IEDs contained power supplies, switches, and detonators, certain of the newer HMEs do not require an electrical detonator but can be initiated by an open flame.