Possible evidence was found in the hours after the blasts at Aldgate Underground station and at the scene of the bus bomb in Tavistock Place.

If the explosive type can be linked to al-Qaeda attacks elsewhere it will give detectives vital clues to the network from which the bombers stemmed.

Investigators will also be examining thousands of hours of closed-circuit television footage from street and station cameras. Police officers will try to spot suspects carrying rucksacks or holdalls on to trains and possibly leaving at another station without their luggage. The study of security film may tell officers conclusively whether they are dealing with suicide bombers or terrorists who planted their bombs and escaped, possibly to strike again.

It appears that there was no intelligence — either through undercover agents or electronic surveillance — that these attacks were likely.

The analysis of CCTV is a basic investigatory step taken in the first hours of a typical murder inquiry. Murder squad detectives have been drafted in from across London to help the Anti-Terrorist Squad.

A Scotland Yard source said of the attacks: “This is a total surprise. We have been priding ourselves on our progress in the last few years but tonight we are having to go right back to basics.

“There is no intelligence; we are just not into that world.”

Forensic science analysis of the bus bomb scene offers the inquiry team “massive potential for recovery”. The likelihood that the bomber died in the blast and the fact that it was above ground raises the chances of finding valuable evidence.

British scenes-of-crime officers are renowned for their ability to recover and preserve evidence. They will divide the area around the attack into sectors and collect every piece of material before reconstructing the scene in an aircraft hangar and making a detailed analysis.

The shattered Tube train carriages will be removed from the tunnels and taken to locations where they can be stripped down and examined.

The bombers, who no one doubts are part of an Islamist group, could have emerged via two routes. The threat that the UK authorities know most about is that which comes from Algerian and North African networks known to have based themselves in London since the 1990s. The al-Qaeda team that carried out the Madrid train bombings had these origins.

But in the past year British police have encountered growing numbers of British-born terrorist suspects — recruited and indoctrinated here but trained overseas.

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