The investigationA huge forensic investigation was stepped up at the four bomb sites in London yesterday with the aim of identifying the bodies of the victims and those responsible for the blasts.
The London Metropolitan police, which is coordinating the operation, dispatched scene-of-crime officers with forensic support staff to secure all of the sites.
"Forensic examination of the scenes is vital because it could save lives in the future," a senior Scotland Yard officer said.
Officers are taking swabs from clothing and surfaces near the blasts in the hope that chemical analysis will reveal residues of the bombs used. Police believe that each of the bombs contained no more than 10lbs (about 4.5kg) of explosive and may have been carried in rucksacks.
At each site the task is different. Plans are being drawn up to remove the entire remains of the bomb-damaged bus to a secure site where forensic experts will attempt to piece it together to help to identify where the bomb was placed.
At Russell Square in Bloomsbury, forensic teams face the gruesome and complex task of gathering evidence from tube carriages deep underground.
Bodies in the front carriage, where the bomb is believed to have detonated, must be removed before any detailed examination can begin. Because of the intense heat and difficulty of working in the tunnel, Scotland Yard is considering ways of extracting the front carriage intact for forensic investigation.
Clues to the bombers' identities may also be found on CCTV footage from the bus and tube stations and from cameras around the city, and investigators are gathering hundreds of hours of tape.
Graham Thomas, a forensic consultant with 20 years' experience, said that large quantities of the bombs should be recoverable from each scene. "The initial blast fragments the bomb, but you can expect to recover about three-quarters of it," he said.
At each of the sites there will be a fingertip search for components of the bombs. The types of materials used, the detonators and casings will help to narrow down who was responsible. Bombers also tend to construct their weapons according to where they learned the technical skill, providing further clues as to their origin.
Teams will also face the task of identifying the victims. According to Sue Black, a forensics expert at Dundee University, in most cases identifying the bodies should be possible using personal effects.
But human remains will have to be pieced together to help return victims to families and also confirm the nature of the explosion and whether any of those recovered were carrying the bombs.
"You need to build up a body map of each person, so you can eliminate people as suspects and return bodies to families," said Prof Black. "But it's vital for other reasons. If you find that victims have their feet missing, it could indicate a device in a bag left on the floor." Experts believe the forensic analysis will go on for some weeks.
"Whenever you miss something, you can't go back to it, so it's very important that the work at the outset is done painstakingly. People want answers now, but it takes time to make sure you get it right," said Prof Black.