The home secretary compared the task to "looking for needles in a large haystack". Intelligence officials said forensic analysis of the bus blown up in Tavistock Square, Bloomsbury, would prove vital if it led to an identification of one of the bombers.
There is widespread speculation that the bomb on the bus was meant for another underground train but went off before the bomber had time to put it in place.
A senior Scotland Yard source said that CCTV footage of the underground could also provide valuable evidence. The police refused, however, to comment on reports that timers had been found, which would be a clear indication that the terrorists were not suicide bombers.
Scotland Yard also dismissed reports from the US that two unexploded bombs had been discovered.
As the police and forensic scientists searched on the ground for the smallest of clues, MI5, MI6 and GCHQ stepped up their surveillance in the hope they would pick up clues in the airwaves.
GCHQ, the government's electronic eavesdropping centre, was said to be "absolutely engaged in the hunt". MI6, the Secret Intelligence Service, and MI5, the domestic security service, were asking their counterparts abroad for help.
Sir Ian Blair, the Metropolitan police commissioner, emphasised the "need for information". That was an appeal on behalf of MI5 as well as the police.
One senior anti-terrorist official pointed to a central concern of the security and intelligence agencies. "The worrying thing is they live to fight another day," he said.
A senior Scotland Yard source said: "We have to be running on the assumption that the people who did this are still out there and they could do it again."
Spanish security forces discovered a second stash of bombs after the Madrid attacks in March last year. The terrorist suspects blew themselves up in a shoot-out with the Spanish police.
Sir Ian said there was so far "absolutely nothing" to suggest the attack on the bus in Tavistock Square had been the work of a suicide bomber, though he added: "We cannot rule it out, it may have been, but it [the device] may have been left on a seat, it may have gone off in transit."
He also said it looked as though the bombs on the tube trains had been on the floor of the carriages. Police sources said the bombs were all left by the carriage doors and were less than 10lbs (about 4.5kg).
Hans Michels, an explosives expert at Imperial College London, said something weighing less than 10lbs would be a "fairly high quality explosive to cause that sort of damage", such as a military type of explosive.
The explosives could have been the size of a wine box or a few books. "If you put something like that on a luggage rack it would be small enough for people not to take notice of it. It could be operated with a timing device or remotely."
Security and intelligence officials said it was too early to say who was suspected of carrying out the attacks and whether they came from abroad or had been living in Britain for some time. "We are getting information all the time," said one intelligence official, "but we don't know if it is potentially useful or not."
However, they do not contest that the operation must have been well planned and that the bombers must have been helped by others, perhaps in "sleeper cells".
Mr Clarke said there was a "strong possibility" that an al-Qaida-linked group was behind the bombings - "because the fact is, the modus operandi, the nature of the events and the website claims, all of these give some substance to that being a strong possibility".
He added: "But that is all it is, it is a strong possibility. Every single possibility remains and I absolutely cannot say we believe it was committed by group X or group Y."
A group calling itself the Secret Organisation of the al-Qaida Jihad in Europe posted a claim of responsibility for the attacks on a website, saying they were in retaliation for Britain's involvement in Iraq and Afghanistan.
The police and anti-terrorist agencies declined to comment on any individuals named as terrorist suspects by other countries, including Spain. "Lots of names are being bandied about," said one official.
Whitehall's Joint Terrorist Analysis Centre last month reduced the threat level from al-Qaida terrorism from "severe-general" to "substantial". It has now reversed that decision.
It made the earlier decision on the grounds that the al-Qaida leadership did not have the ability to order a coordinated attack in Britain. It said many of its current concerns focused on individuals or groups "only loosely affiliated to al-Qaida or entirely autonomous".
The security and intelligence agencies are also expressing concern that extremists trained in Iraq will come to - or return to - Britain with new bomb-making skills. There was speculation last night that at least one man had returned, and was responsible for making the bombs for the attacks in London.
