Initial reports all suggested that the explosives used were high grade, possibly military explosives. But after the police searched 18 Alexander Grove it was widely reported they had found TATP.
See government references to the explosives in the reports analysis
CNN: Technical data and witness accounts suggest the bombs contained synchronized timing devices and were probably not triggered by suicide bombers, police said, adding that the bombs were composed of "high explosives" and probably not homemade material. ('Level of sophistication')
Independent: Christophe Chaboud, head of the French Anti-Terrorism Co-ordination Unit, told Le Monde newspaper that the explosives used in the bombings were of " military origin"
Observer: The fact that the type of explosives used was not hand-made, but small 10lb commercial high explosives, probably linked to a timing device, suggests a level of sophistication and financing.
IHT: They said the material used in the bombs was similar to the kind manufactured for military use or made for highly technical commercial purposes, such as dynamite used for precision explosions to demolish buildings or in mining.
IHT: On Saturday, Andy Hayman, who is in charge of Scotland Yard's antiterrorism unit, announced that the four bombs set off in London each contained less than 4.5 kilograms of explosive material.
Police discovered explosives in a hire car and also in a house in Leeds where they reportedly found quantities of TATP. Shortly after this we were told that police had found traces of TATP at the bomb sites from the 7/7 attacks and also at the house raided in Leeds. (janes)
Terrorists do not usually just leave explosives for their enemies to find. (bombs in car - Times)(explosives in house - Independent)
There were many conflicting reports as to whether timing devices were actually found. The NYPD seemed to think it was mobile phone detonators, but the story that the bombs were detonated via a button appears to of stuck.
Independent: The victims' wounds suggested that the explosives, which were "not heavy but powerful", had been placed on the ground, perhaps underneath seats.
DC Andy Hayman: 'was not able to say how the devices had been detonated but confirmed that each device on the trains was placed on floor of a carriage, and on the bus a device was placed on the floor or seats.'
Independent: Explosive used in bombs 'was of military origin'
ITN news: Explosives expert opinion - video