LONDON (AP) - Almost anywhere Londoners go in public, chances are they're being tracked by cameras.
Now the British capital's ubiquitous closed-circuit TV cameras may hold the key to determine who was behind Thursday's series of terrorist strikes on the London Underground and a double-decker bus. Thousands of cameras watch the subway system alone, and investigators have used the footage in the past to solve crimes.
London's train stations are monitored by 1,800 cameras, and there are more than 6,000 watching the capital's Underground subway network. Cameras also have been installed on some London buses.
On top of that, many people carry pocket-size digital cameras and camera phones, and some footage from such sources, inside subway cars, had already found its way to British TV by afternoon.
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.
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.
Charles Shoebridge, a security analyst and former counterterrorism intelligence officer, 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. If investigators can determine where bombers boarded the trains they can check records of cell phone calls made in the area.
"Clearly we've had considerable success in the past using closed-circuit television footage in order to trace the movements of the people involved" in various crimes, Deputy Assistant Commissioner Brian Paddick said after Thursday's attacks.
"That will be one of our first priorities, as well as securing whatever forensic evidence we can secure from the various scenes," Paddick said.
An estimated 4.2 million cameras - largely concentrated in London and other major cities - observe Britons as they go about their daily business, whether they're waiting for a bus, riding a train, lining up at a bank or parking a car in a public garage.
The phalanx of security cameras has earned Britain a reputation as a world leader of surveillance, and has led to criticism that the constant monitoring takes the Orwellian concept of "Big Brother" to an extreme. It's widely estimated that the average Briton is caught on various cameras up to 300 times on a normal day.
Critics also contend the system is of limited use to counter terrorism because the monitoring screens at command posts aren't closely watched in real time, meaning the images rarely are used to stop a crime in progress - only help authorities investigate one after the fact.
Last year, closed-circuit video helped British authorities in several high-profile cases, including a 12-year-old boy who robbed a store at gunpoint; the disappearance of a doctor; attacks by a serial rapist; a father and son hit by a train; a rash of school laptop computer thefts; and a soccer riot.