The bombs used in the July 7 attacks on the London transit system were peroxide-based, carried by the bombers in coolers the morning they struck, police said Tuesday.
Investigators said they found two unexploded bombs in a car the suspected attackers left at a train station north of the capital. Those devices were also made of peroxide and were surrounded by nails, police said.
Police did not immediately specify how they knew the bombs that exploded aboard three London subways and a bus were made from peroxide, according to the AP.
There had been speculation for weeks that the explosives were peroxide-based, but police had not confirmed it publicly.
Peroxide-based explosives degrade at room temperature and must be kept cool until used.
Peter Clarke, head of London's Metropolitan Police anti-terrorist branch, said the bombs had been "effectively made" and that whoever made them had "done some good research and had been well trained."