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