A massive police hunt began yesterday for what now appears to be a small al Qaeda cell responsible for four bombs and the death of more than 50 people, as the friends and families of those still missing scoured hospitals in the hope of finding their loved ones.

Early speculation that suicide bombers were involved appeared to be receding, with evidence that the tube station bombs had been placed on the floor nest to the door of the carriages. Anti-terrorist sources said the main anxiety was that the bombers were still at large and could strike again.

Underneath London’s streets, hundreds of officers and emergency workers continued digging out the remaining bodies from the tube carriages, and searching for clues that could lead to the prosecution of those responsible. Conditions were difficult and dangerous.

On Friday night, 49 people were confirmed dead with up to 20 more bodies still in the underground train at Russell Square where the blast seriously damaged the tunnel.

Sir Ian Blair, the Metropolitan police commissioner, said the nature of the attack, the worst terrorist attack on mainland Britain, was becoming more clear. Four bombs, weighing less than 4.5kg each, were probably carried in backpacks and placed on the floor of the three underground carriages and on the seat or floor of the number 30 bus that was blown apart in Tavistock Square.

“There is nothing to suggest that there was a suicide bomber,” said Sir Ian, “but nothing can be ruled out.” The bombers, he admitted, could be at large in Britain, already out of the country, or dead. Nothing could have been done to prevent the attack, he said. “No intelligence service is perfect. This is an imperfect world and it is an imperfect science.”

The leads now being pursued will come from closed circuit television tapes, cell phone calls made in the area of the bombs, and from what the police hope will be tip-offs from people who have noticed that a lodger is missing or that a lock-up garage is deserted.

Attention has focused on the No 30 bus and the possibility that the bomb there was in transit to another tube train and went off prematurely, killing 13 people. No arrests had taken place by last night.

“Our total effort today is focused on identifying the perpetrators and bringing them to justice,” said the home secretary, Charles Clarke. “That is, of course, the number-one preoccupation that the police and the security services have at this moment.”

London was almost back to normal yesterday. Most of the public transport system was functioning and Sir Ian urged everyone to be behind their desks at the start of next week. “This is business as usual on Monday,” he said. “We go on.”

For the families and friends of those now confirmed dead or unaccounted for, this will not be the case. In scenes reminiscent of 11 September, friends and families carried photographs of the missing as they went from bomb site to hospital to television studio, or tied the pictures to railings. The images acted as proof of the extraordinary diversity of the city, and the names provided evidence that those affected came from all ethnic and religious groups.

One family hired a private detective in their search, others faxed the hospital with photos. More than 104,000 calls have been made to the emergency information number.

“These are seven days that no-one will ever forget,” said Ken Livingstone, London’s mayor, as he and the commissioner sat shoulder to shoulder and delivered an appeal for information. Mr Livingstone, who, like New York’s mayor Rudy Giuliani, in the wake of 11 September, managed to articulate his city’s dismay and resolve, said that there had been no panic.

He proclaimed his belief that London, in all its diversity and with a population that speaks more than 300 languages, “typifies... the beauty of the human race”.

The Queen and the Prince of Wales visited the wounded in St Mary’s Hospital, Paddington. “It’s been one of the things that many of us have dreaded for a long time,” said Prince Charles. “What I can never get over is the resilience of the British people who have set us all a fantastic example of how to recover.”