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Thursday’s attacks on London’s tube network and on a double-decker bus in Hackney bear the hallmarks of a botched operation. As more information emerges, the authorities in London have confirmed that at least three of the devices isolated on Thursday were of a similar size to those detonated by Islamic extremists during the 7/7 attack on London earlier this month. The fourth device appears to have been slightly smaller. These devices consist of three main components. The first is a battery powered timing power unit or TPU which initiates the detonation sequence electrically. In recent times, mobile phones have been adapted by terrorists to function as TPUs. When activated, the TPU heats a metal filament within the second component - the detonator. Detonators contain a small amount of highly sensitive and volatile high explosives such as lead styphnate or mercury fulminate. When triggered by the TPU, the detonator generates a small explosion which provides the shock wave necessary to ignite the third component of the bomb – the bulk charge. The bulk charge normally consists of a plastic explosive. The types of plastic explosive favoured by contemporary terrorist and resistance groups are sometimes of commercial origin but more often of military specification such as semtex, C4, P4, HDX or RDX. All of these explosives are nitrogen-based and contain nitro-glycerine or nitrocellulose. There is some suggestion that the bulk charge in the London attacks may have been home-made, or improvised in a terrorist bomb-making facility. Even a small amount of such nitrogen based explosives, between 2 to 5 kilos would be more than capable of causing the type of death and destruction as seen in the London attacks on the 7th of July.
Clonan, T., 2005: London's Suicide Bombers: Botched Operation, Dublin: The Irish Times.