British investigators — skilled at anti-terror work from decades of Irish Republican Army bombings — are at the beginning of the daunting task of finding those behind the bus and subway explosions that killed at least 50 people and injured 700 in London Thursday.
Time may not be on their side.
Three weeks after bombs struck four Madrid commuter trains last year, police found some of the plotters in a safe house with more explosives, apparently planning fresh attacks.
Home Secretary Charles Clarke, Britain's top law enforcement official, said investigators were concerned that the while the London bombers are still at large, they could strike again.
"That is of course the No. 1 preoccupation that the police and security services have at this moment," he told British Broadcasting Corp. radio.
Charles Shoebridge, a security analyst and former counterterrorism intelligence officer, said another attack was very likely, "because there's no reason for them not to, they've broken their cover," he said. "They will now try to exploit whatever freedom they have left" to kill again.
CBS News Correspondent Jim Stewart reports
Stewart reports investigators now suspect all the bombers used what they called the "step-on, step-off" delivery system of dropping off a bag and quickly leaving.
Sir Ian Blair, commissioner of the Metropolitan Police said no evidence suggested that the attacks involved suicide bombers but that officials hadn't ruled out the possibility.
Assistant Police Commissioner Andy Hayman said officials believe the bombs were placed on the floors of the three subway cars that were hit. He said the initial investigation suggests that each bomb had less than 10 pounds of explosives.
Police have said they can't confirm the authenticity of a claim of responsibility from a group calling itself "The Secret Organization of al Qaeda in Europe."
Clarke said investigators were examining the claim and it was "something we certainly take seriously."
British Foreign Secretary Jack Straw and many experts and have said the bombings bore some of the hallmarks of al Qaeda — several coordinated, simultaneous hits at symbolically charged targets.
However, al Qaeda has been under intense scrutiny since the Sept. 11 attacks and is believed to be far more loosely organized than it once was. Low-level operators may be able to carry out attacks without direction from the network's leaders.