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 more than 50 people and injured more than 700 in London on 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 BBC 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."
"They will now try to exploit whatever freedom they have left" to kill again, he said.
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 that it was "something we certainly take seriously."
The attacks bear all the hallmarks of al-Qaeda and the cell that was probably behind them is still at large, London's police chief Ian Blair said yesterday.
"This has all the hallmarks of al-Qaeda ... but we are in the beginning of a very complex and lengthy investigation and there is nothing specific that I am aware of," he said.
"There is likely to still be a cell. Whether these people are still in the UK is a question. We must remain vigilant. It is a national issue, not just for London," he said.
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.
Police have found nothing to suggest Thursday's attacks were caused by suicide bombs, Blair said yesterday.
"Nothing at this stage can be ruled out," he told reporters.
"It may have been that it was a bomb left on a seat, a bomb that went off in transit. These things are still open to investigation," he said in reference to a device that went off on a bus.
"Continuous reference to a suicide bomber is unhelpful because it is unproven," Blair added.
It also 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.
In the attacks on four commuter trains in Madrid, which left 191 dead, the bombers left backpacks aboard the trains and used cellphones to detonate them.
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