The search continued Friday for the terrorists whose work killed over 50 people and injured more than 700 in a series of explosions on the London public transportation system.
London police are focused on preventing another attack
Investigators scoured the scenes of the explosions for clues to the identity of Thursday's bombers in an attack ministers said bore the hallmarks of the Islamic militant al Qaeda network.
"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," said Home Secretary Charles Clarke.
British officials were examining a claim of responsibility by a previously unknown group calling itself the Organization of al Qaeda Jihad in Europe, the government said on Friday.
In a statement posted on the Internet -- but which has not been authenticated -- the group said the attacks were "in response to the massacres carried out by Britain in Iraq and Afghanistan." They threatened similar attacks against Italy, Denmark and other "crusader" states with troops in Iraq and Afghanistan.
"The Web site claim is a serious one, so we will look at that very closely but we haven't eliminated any alternative explanations. We're looking at everything very widely," British Home Secretary Charles Clarke told Sky News television.
Suicide attack speculation
Although anti-terrorist investigators said it was too early to tell, several British newspapers speculated that the attack on the number 30 bus had been the work of a suicide bomber. Andy Hayman of the London police specialist operations branch, said the bombs had held 10 lbs. (4.5 kg) of explosives and could have been carried around in a backpack, Reuters reported. The New York Times wrote that they had been detonated by a timing device.
The synchronized blasts, detonated without warning, occurred within minutes of each other, in an eerie echo of last year's Madrid train bombings, which were blamed on Islamic extremists linked to al Qaeda.
Spain said Friday it had sent security experts to help in the London investigation.
Britain's Prime Minister Tony Blair
Prime Minister Tony Blair, during a press conference at the end of the Group of Eight summit in Scotland on Friday, rejected the possibility that the failure of police or intelligence services could have allowed the attacks to take place.
"My opinion is that those people who kill the innocent and cause such bloodshed, that they are responsible and they are solely responsible," he responded to a journalist's question, prompting a stony silence.
Rescue services had trouble reaching an unknown number of bodies trapped in the underground system. They feared the tunnel could collapse while recovering the dead, which one police source suggested could number more than ten, Reuters reported. Police said no more survivors were trapped underground.
The police bolstered their presence on the streets of the capital to reassure wary commuters on Friday, as governments around the world beefed up security in the wake of Thursday's blasts, the worst terror attack ever in Britain.
Sniffer dogs and extra police patrolled London Underground stations as service across the rail network, the scene of three devastating explosions, slowly returned to normal.
The blasts tore apart packed underground trains and a further bomb peeled off the top a double-decker bus on Thursday morning, as terror replaced the euphoria of a day earlier, when London was named host of the 2012 Olympic Games.
Flowers left near the entrance of King's Cross Tube station
Above the ground, the agony of loved ones of those missing and still unaccounted for had begun.
"Missing, with school friends at a party or concert, Miriam Hyman," read one of a series of hand-made posters at King's Cross station.
Hyman's father, John, said he had spoken to his daughter by mobile phone shortly after the bomb, and feared she had left the station to catch the ill-fated bus.
"It may be that she is in hospital suffering from shock or has lost her purse with her identification in it," he said.
Adding to the sense of tragedy, bunches of flowers fitted with messages of sorrow were piled near the entrance of the Tube station.
"It's got nothing to do with politics or religion," said Atiya Fatiha, whose daughter often uses the Picadilly line that runs through King's Cross station, near to where a bomb exploded on a train killing at least 21 people. "These were just ordinary people going to work," said Fatiha, with tears rolling down her cheeks as she lay a wreath down with about a dozen more, on fencing near the main entrance to the Tube, which remains still shut.