LONDON (Reuters) - Police scrambled through dangerous rail tunnels deep underground on Friday to hunt for clues and retrieve bodies after suspected al Qaeda bombers killed more than 50 people in London rush-hour blasts.
A day after four bombs tore through three underground trains and a red double-decker bus, commuters headed to work again on London's battered transport network, some fearful, some defiant, undeterred by the knowledge it might happen again.
"My granddad called me last night and told me I had to go to work today," said Sally Higson, 36. "He's 89. He lived through the war and said it was important to carry on as normal."
The attacks -- which ministers said bore the hallmarks of the Islamic militant al Qaeda network -- were London's deadliest in peacetime and disrupted a summit of the Group of Eight (G8) industrialized countries in Gleneagles, Scotland.
London police chief Ian Blair said more than 50 people were killed in the blasts and 700 wounded.
He said the final death toll was not yet known. Police had yet to reach one of the bombed underground carriages in central London as the surrounding tunnel was unsafe.
Blair said no survivors were trapped underground and the task now was to retrieve bodies. Andy Trotter of the British Transport Police said the number of bodies still trapped was not known, but one police source said it could be more than 10.
"This was a crowded tube train at rush hour in central London with several hundred people on board," Trotter said.
Andy Hayman, of the London police specialist operations branch, spoke of the "extreme circumstances" under which rescue services were working, saying they faced the hazards of tunnel collapse, vermin and "dangerous substances" in the air.
"Just imagine an explosion that far into a tunnel," he said. "I think we can all respect the sort of things our people are actually confronting."
A maintenance worker, who did not want to be identified, said he had reached the site early on Friday and described "awful" scenes, with several bodies in the carriage.
"We got up to the carriage, although it was very dark there at the time," he told Reuters. "The smell was awful."
Hayman said the bombs were believed to have contained up to 10 lbs (4.5 kg) of explosives and could have been carried onto the trains and bus in backpacks.
Police said they had no specific intelligence warning of the attacks. Home Secretary Charles Clarke said the blasts "came completely out of the blue."
The New York Times said timing devices rather than suicide bombers set off the explosions and Blair stressed there was nothing so far to suggest suicide attacks.
MORE BOMBS FEARED
Fears of more attacks kept commuters and markets jittery.
"We have to have ... maximum consideration of the risk of another attack and that's why our total effort today is focused on identifying the perpetrators and bringing them to justice," Clarke told BBC radio.
He said investigators were examining a statement from the "Secret Group of al Qaeda's Jihad in Europe."
The e-mail statement by the previously unknown group said: "Our mujahideen have carried out a blessed invasion in London and here is Britain now burning with fear and terror ... We have repeatedly warned Britain and have kept our promise."
Clarke said: "We monitor now very intensively a wide range of Web sites ... and this one and their claim is something we certainly take seriously."
An Internet statement from another group, calling itself the "Organization of al Qaeda - Jihad in the Arabian Peninsula" praised the attacks and said Rome would be targeted next.
Britain and Italy backed the U.S.-led war in Iraq, and London had been on high alert since al Qaeda's Sept. 11, 2001, attacks on the United States and 2004 Madrid train bombings.
The blasts battered financial markets, but oil prices recovered to near $61 on Friday as analysts said the attacks were expected to have a limited impact on the global economy and oil demand. British shares also recovered their losses, but sterling slid further against the dollar.
"There's still a bit of a terrorist risk premium in the market," said a strategist at a London bank. "It would make a difference if we knew for sure that these were suicide bombers, rather than an active cell on the loose."
The Islamic Human Rights Commission warned London Muslims to stay at home, fearing a backlash against the community. The Muslim Council of Britain, which represents 1.6 million Muslims, called for prayers for the victims.
Thursday's scenes of shocked and bloodied commuters were in stark contrast to the jubilant crowds who took to the streets on Wednesday after London was awarded the 2012 Olympic Games.
Clarke confirmed intelligence chiefs had reduced the threat level only last month following Britain's May 5 election but said there was no indication a higher alert level would have detected the bombers.
