The police operation surrounding the 7 July London bombs is "enormous" and "is still very much a live investigation", the government's official report said.
It said the bombs were constructed using materials which were readily available and which required "little expertise" to turn into the explosive devices.
The group, consisting of outwardly unremarkable young men, was motivated by "fierce antagonism to perceived injustices by the West against Muslims" and a desire for martyrdom.
But the report, released on 11 May 2006, said many details about how the four bombers were radicalised, how the attacks were carried out and whether others were involved were still to be discovered.
The extent of al-Qaeda involvement in the attacks, which killed 52 people, also remained unclear.
Investigators said there was no evidence of a fifth bomber, however.
Police were checking more than 12,500 statements, 26,000 items - including 5,000 which were being forensically examined - and 6,000 hours of CCTV.
The "real difficulty" in identifying potential terrorists was that there had been little outward sign of the potential for violent extremism among the men, the report said.
With the possible exception of Germaine Lindsay "there is little in their backgrounds which mark them out as particularly vulnerable to radicalisation", it said.
The 7 July bombs killed 52 people and injured more than 750
Police forensics teams are examining thousands of items
The bombers had not aroused the suspicions of those who knew them