For some years, the British capital has been rigged up with one of the world's most comprehensive closed-circuit television (CCTV) systems which have allowed police to track and snare countless criminals.
Just after Thursday's blasts, investigators seized thousands of video tapes and digital recordings which are currently being ploughed through frame by frame.
"There's a large number of CCTV tapes we're going to take and examine," said Andy Hayman, London's Metropolitan Police assistant commissioner of specialist operations.
Met deputy assistant commissioner Brian Paddick added: "We've had considerable success in the past using CCTV footage to trace the movements of people implicated in criminal activity. That will be one of our first priorities."
With around 1400 cameras, including several high definition cameras, installed in the Underground's tunnels and platforms, some 8000 on board buses and thousands on the streets, there is every chance one or more of those behind Thursday's attacks on London transport will be caught on camera.
According to a recent university study, which estimated a total of seven million CCTV cameras in Britain, each Londoner is filmed 300 times per day on average.
"The CCTV system can be enormously important in the pursuit of an inquiry," said the director of CCTV User Group, which brings together some 700 towns and cities, universities and shopping centres using surveillance cameras.
UK Police say checking footage
is one of their top priorities
"In the investigation into the attack on the Admiral Duncan pub [by a neo-nazi on the London gay bar in 1999, killing three], the police started by removing all the recordings in the area," Peter Fry said.
"They spotted a guy with a blue bag like the one the bomb was in, then shortly afterwards without the blue bag.
"They looked at the tape from the Underground cameras and found a very high-definition shot of his face. They showed his face on television and people recognised him.
It is impossible to know the exact number of surveillance cameras in London and across the country.
Figures vary from one source to another due to the number of private systems which are not officially counted. However, their footage can be removed via a simple request by the police, Fry explained.
The first cameras, which filmed in black and white, were installed in 1961 in the London Underground.
Nowadays, waves of ever-improving systems hit the market such as computers which can lip-read, recognise faces or detect suspect behaviour, for example by recognising people walking against the traffic flow in a subway, or an object which remains immobile for several minutes, which can trigger an alert.