Investigation
It was immediately apparent that the explosions were the result of a terrorist attack, but it was initially unclear whether the explosions were a serious attempt to repeat 7 July bombings or were merely a symbolic attack or hoax intended to cause panic rather than mass casualties. The explosives used by the bombers consisted of Chapatti flour powder mixed with liquid hydrogen peroxide, detonated by a booster charge. This was the same explosive mixture used by the bombers in the 7/7 bombings two weeks earlier. It was later confirmed that substantial improvised explosive devices capable of causing significant numbers of casualties had in fact been involved, but had failed to explode. The explosions were caused by detonators which failed to detonate the main explosive charge. Police later disclosed that some of the devices used had survived the explosions and were available for forensic investigation.
Read more about this topic: 21 July 2005 London Bombings