Investigators Face Daunting Task

Hours of closed circuit television footage to scrutinize, tons of wreckage to sift through, tiny traces of explosives to analyze.

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.

Jordanian-born terror leader Abu Musab al-Zarqawi, part of the al Qaeda group now based in Iraq, has indicated a desire to reach beyond that country and hit Western targets. Investigators could examine the possibility of his involvement.

It wasn't immediately clear how sophisticated the London bombers' explosives or detonators were.

That could be an indication of how well-linked they are to international networks, since powerful plastic explosives are far more difficult to obtain than crude, less incendiary materials. Madrid's bombers claimed far more victims than London's in their train attacks.

Another unanswered question is whether any of the London bombings — one on a bus and three on the subway — may have been carried out by suicide bombers, as is often the case in Israeli bus bombings and in Iraq. Police say they don't yet know and Clarke said it was possible.

In the attacks on four commuter trains in Madrid, which left 191 dead, the bombers left backpacks aboard the trains and used cell phones to detonate them.

The phones gave investigators a lucky break that led them to some of the attackers. One bomb failed to go off, and the subscriber identity card inside that phone eventually led investigators to the suspects, although they haven't found the plot's masterminds.

Police in London may get a break like that too, but they also have a lot of hard slogging ahead of them.

London is crammed with closed circuit television cameras — 1,800 monitor its train stations, 6,000 watch the Underground network and some buses also have cameras.

Shoebridge said detectives would have to watch thousands of hours of tape — slowly and carefully.

Investigators will try to find on tape the point at which bombs were placed and then trace back the movements of the person they identify as the bomber, an arduous task that could involve hundreds of cameras, Shoebridge said. Most of London's Underground cameras are in stations, not cars.

Shoebridge said investigators would also check records of mobile phone calls made in the bombed areas just before the explosions, a job that might be difficult in the case of the Tube attacks unless investigators determine where bombers boarded the trains.

They'll likely look at the ways someone might obtain explosives, or the means to make them, talking to chemical suppliers and others who might provide leads.

Forensic evidence will also be key. If any of the perpetrators were suicide bombers, there will be body parts to examine for clues. If not, detectives will search for DNA or fingerprints.

They'll also have to examine recent intelligence — including the phone and e-mail intercepts routinely collected as part of anti-terrorism work — to see if any clues were missed or if any of the communications contain information that looks significant in hindsight, Shoebridge said.

Old interviews with informants will be re-examined and new ones conducted.

Authorities will have to identify "whatever failings exist, if any, in the intelligence system that allowed this attack to take place, because it is an intelligence failure," Shoebridge said.