LONDON — Struggling against heat and rancid air, emergency crews yesterday tried to reach bodies entombed in London's famed Underground as the death toll in Thursday's terrorist bomb blasts was certain to climb past 50.

Forensic specialists searched through the bloody, charred debris of three other bomb sites as they tried to determine who carried out the coordinated attacks. Investigators said each of the bombs used in the blasts appeared to contain no more than 10 pounds of explosives and was small enough to be carried in a knapsack.

The bloodiest attack on British soil since World War II paralyzed London's vast transportation network, disrupted a summit of the leaders of the world's richest countries, being held in nearby Scotland, and expanded another front in global terrorism.

As this city fought to heal itself, desperate families embarked on searches for missing relatives.

In smaller numbers than usual, Londoners went back to work yesterday, boarding double-decker buses and Underground trains like those hit in Thursday's bombings, which also wounded some 700 people. Scores remained hospitalized with burns, cuts and severed limbs. The official death toll stood at 49.

Police were on patrol outside many of London's mosques during Friday prayers, bracing for a potential backlash against Muslims. The British government said it suspects Islamic terrorists were behind the bomb blasts, and the Web-site assertions of a group claiming to have committed the acts on behalf of al-Qaida were gaining currency.

Counterterrorism officials vowed to wage a manhunt of unprecedented proportion. Police were reviewing scores of surveillance tapes taken from cameras in the subway system and interviewing hundreds of witnesses.

It "is blindingly obvious [that] there is likely to still be a cell" of terrorists active in the United Kingdom, Metropolitan Police commissioner Ian Blair said.

"We must remain vigilant," he added, calling on the public to cooperate in the investigation.

The driver of the No. 30 bus on which 13 people were killed revealed yesterday that his bus had been diverted in the chaos following the earlier blasts. That might have thrown off the timing for the person believed to have been transporting the bomb, causing it to explode before the intended destination.

There has been speculation over whether the bus explosion, in contrast to the blasts on the subway cars, involved a suicide bomber, which would be the first such incident in Britain.

But investigators say a timing device may have been found in the bus debris, suggesting that the bombing was not meant to be a suicide mission.

European law-enforcement officials briefed by the British said investigators may have recovered body parts of a suspected bomber, which would be an invaluable find in determining the identity of the culprits and tracing their networks.

"The British think they probably have the corpse of a terrorist among the dead on the bus," a senior European official said. "They are pretty convinced that it is one of the suspects. And that the explosion may have been involuntary or ahead of schedule."

Fearing the collapse of a tunnel on the stricken Piccadilly Line, authorities said yesterday they still had not removed the tangle of bodies in the bombed train more than 100 feet below street level between Russell Square and King's Cross station. They said they were confident, however, that there were no additional survivors.

The smashed carriage is at least 500 yards within the tunnel, making access difficult. Work in the hot, cramped, rat-infested space can be sustained only for short periods. Emergency crews had to shore up the roof and walls of the tunnel to continue the search and recovery operation.

"They are very challenging scenes," said Andy Hayman, assistant police commissioner.

Once work inside the Russell Square train is completed, the death toll is expected to increase significantly but not reach "triple digits," Blair added. The police officials spoke at a news conference.

Hayman said timing devices were discovered in the debris. The bombs appeared to have been placed on the floor of the subway cars, suggesting that they were planted and not attached to suicide bombers.

The evidence so far suggests that terrorists used bombs smaller but similar to the ones in attacks on the Madrid, Spain, train system last year, in which 191 people were killed. Authorities traced the Madrid bloodshed to a group of Islamic extremists primarily from Morocco. Most of the suspects were caught or killed within weeks of the bombings. However, several allies of the cell remain at large, including at least one suspect who has had British residency documents.

A team of Spanish detectives traveled to London yesterday to help with the investigation.

More perplexing in the London carnage is the attack on the double-decker bus. After saying Thursday that only a few people were killed when the blast ripped off the second level of the bus, authorities yesterday said they had recovered 13 bodies from the crumpled red vehicle. Many of the bodies were badly mutilated, paramedics on the scene said.

There has been speculation that in the bus attack, the person transporting the bomb ended up killing himself inadvertently when the device blew up prematurely, or because he was delayed by the detour of the bus.

The bus blew up nearly an hour after the first subway blast; all three subway explosions were triggered within 26 minutes.

Richard Jones, who said he got off the bus just before the blast, told British media he saw a man in his mid-20s fidgeting with a bag.

"He was standing next to me with a bag at his feet, and he kept dipping into this bag and fiddling about with something," Jones, an information-technology consultant, told the BBC. Jones, who gave a statement to police, said he left the bus because it was moving too slowly.

Investigators said the wreckage of the bus could prove a rich environment for clues because it is above ground and accessible and because one of the dead might have been the bomber. Some analysts said the bomber could have been thwarted from entering nearby Euston station, which was shut down after the first blasts, and boarded the bus to blow it up or ride to another potential target.

"Perhaps in panicking he jumped on a bus and tried to turn off the device," said Dominic Armstrong, director of research and intelligence for Aegis Defense Services, a London-based private security firm. "It's too anomalous to have been a concerted attack on London buses."

Britain, like Spain at the time of the Madrid bombings, supports the Bush administration's war in Iraq, making it a target for some extremists.

Responding to allegations that the bombers exploited a security lapse, British Home Secretary Charles Clarke denied that authorities had received a specific warning just before Thursday's blasts.

"We are obviously looking very closely at all our intelligence to see if anything was missed," Clarke said. "But, in fact, we don't think anything was missed and it simply came out of the blue."

For Londoners recovering from the blasts yesterday, the motives behind the violence took a back seat to grief.

Despondent families made the rounds among hospitals, bomb sites and back to hospitals in search of missing relatives.

The smiling faces of the missing were beginning to crop up on posters and in televised appeals: "Have you seen him?"

Meanwhile, Queen Elizabeth, her son Prince Charles and his wife, Camilla Parker Bowles, spent the day visiting the scores of wounded, as well as those who saved the wounded. "Those who perpetrate these brutal acts against innocent people should know that they will not change our way of life," the queen said in a brief speech to the hospital staff. "I want to express my admiration for the people of our capital city who in the aftermath of yesterday's bombings are calmly determined to resume their normal lives. That is the answer to this outrage."

Los Angeles Times writers Sarah Price Brown, John Daniszewski, Ken Silverstein, Janet Stobart, Carol J. Williams and Sebastian Rotella contributed to this report, which was augmented by The Washington Post.