Hours of closed-circuit television footage to scrutinize, tons of debris to sift through, small traces of explosives to examine.
British investigators — their skills honed by anti-terror work from decades of Irish Republican Army (search) bombings — find themselves at the start of a daunting task to track down those responsible for Thursday's deadly explosions in London.
"There is real passion now in the police to make arrests quickly before further attacks can be carried out," said Charles Shoebridge, a security analyst and former counterterrorism intelligence officer.
Based on evidence recovered from the rubble, investigators believe some of the bombs were on timers, a U.S. law enforcement official said on condition of anonymity because the investigation was ongoing.
Investigators doubted that cell phones — used to trigger last year's train bombings in Madrid — were used to detonate the bombs Thursday because reception is spotty in the Underground's tunnels, the official said.
Foreign Secretary Jack Straw (search) said Thursday's bombings — which came the day after London won the bid to host the 2012 Olympics and as British Prime Minister Tony Blair (search) prepared to open a G-8 summit in Scotland — have the "hallmarks of an Al Qaeda-related attack."
Shoebridge said a second attack was 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, because it is likely they will eventually be caught, Shoebridge added.
A group calling itself "The Secret Organization of Al Qaeda in Europe" said in an Internet statement that it staged the blasts in retaliation for Britain's involvement in Iraq and Afghanistan. Police said they couldn't confirm the authenticity of the statement.
A senior U.S. counterterrorism official said the claim is considered "potentially very credible" because it appeared on a Web site that in the past has been used for extremist postings, the message appeared soon after the attacks and does not appear rushed.
The name of the group is also similar to the formulation used by Iraq's most wanted terror leader, Abu Musab al-Zarqawi (search), who calls his organization Al Qaeda between the Two Rivers.
"We know that based on things that have been made public that Zarqawi has made threats not just in terms of the Iraqi theater, but also outside the Iraqi theater," said the official, who spoke on condition of anonymity because of the sensitivity of the information.
One issue hampering the investigation is fear that the tunnels themselves may have been damaged in the blasts, the official said. It could be some time before engineers determine the tunnels are safe enough to allow investigators to collect evidence, the official said.
The U.S. counterterrorism official said there was no confirmation "at all" so far that homicide bombers were involved.
The official said British authorities identified suspicious packages Thursday and detonated at least one of them in a controlled explosions. It is not yet clear whether the contents of those packages were dangerous.
The cell phones used in the March 11, 2004, attacks, which left 191 dead in Madrid, led investigators 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.
After New York's World Trade Center was attacked with a truck bomb in 1993, one of the conspirators gave investigators a hand by trying to retrieve a deposit he'd put down on the vehicle destroyed in the blast.
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 in its train stations, 6,000 in the Underground network and some on buses.
Shoebridge said detectives will 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, then trace back the movements of the bomber, a task that could involve hundreds of cameras, Shoebridge said. Most of London's Underground cameras are in stations, not subway cars.
Shoebridge said investigators also will check records of cell phone calls made in the bombed areas just before the explosions, a job that might be difficult if investigators can't determine where bombers boarded the trains.
Forensic evidence will be key. If any of the perpetrators were homicide 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.
In the end, 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.