The long, intensive sifting of evidence to identify bombers

Teams of forensic science specialists and anti-terrorist officers are working round the clock to gather evidence about the four devices detonated on the Tube carriages and a bus.

The make-up of the devices, and the way they were detonated and planted will provide crucial evidence in helping catch the suspected al-Qa'ida bombers. The investigation is also examining whether the device on the bus was detonated by a suicide bomber.

Police have begun what they described as their biggest search through the footage from surveillance cameras to try to identify the terrorists. As well as trawling through hours of video from cameras on the bus and Tube trains hit in the blasts, they will also have to view film taken from cameras on every building and train station that the trains and bus passed on Thursday.

Scotland Yard's anti-terrorist branch has already established that the four bombs each contained up to 10lb of high explosive, such as TNT and Semtex.

Each of the crime scenes has been cordoned off while forensic experts swab the area, the clothing and bodies of the dead and injured, looking for traces of explosive, so they can identify what material was used. They are also carrying out fingertip searches for any fragments of the bombs and the containers they were hidden inside.

Every single scrap found at the scene will be collected, photographed and stored. The bus and the three carriages will also be removed and taken to the police's forensic explosive laboratory where they will be taken apart and analysed for clues.

The police have yet to establish how the devices were detonated, whether it was a timer, a fuse, a remote device such as a mobile telephone, or if it was in person in a suicide attack.

At this stage the most likely system used was some form of timer, but nothing has yet been recovered. The Metropolitan Police did consider having the mobile phone network closed as soon as they realised there was a bombing campaign in case they were used to set off explosives by remote control, but decided this would cause panic and was unnecessary. In any case, some mobile phones do not work underground.

Fire Service experts are helping the police with the investigation and will advise on the burn patterns found on the dead, which give an indication of where the explosion originated.

Early analysis of the four blast sites has revealed that in the explosion between King's Cross and Russell Square the bomb was in the first carriage in the standing area by the first set of double doors.

For the Edgware Road blast, the device was placed in the second carriage of the train in the standing area by the first set of double doors. In the Liverpool Street and Aldgate explosion, the device was in the third carriage of the train, although it is not clear where it was placed.

On the bus, the bomb was almost certainly on the top deck, on the floor or a seat, which is surprising, because in similar attacks in the Middle East explosives are usually detonated on the lower deck by the stairwell to cause maximum casualties. Commenting on the size of the bombs, Assistant Commissioner Andy Hayman, who is head of the Met's specialist operations, said: "We would expect that to be in a rucksack-type carrier."

The use of a high explosive, rather than a home-made fertiliser bomb, may indicate that the terrorists were from outside Britain where it is much easier to get access to that type of material. Pipe bombs may have been the devices used in the blasts, a forensic expert said yesterday. Dr Niamh NicDaeid, from the University of Strathclyde, said a plastic explosive could have been used. Pipe bombs and plastic explosive can both be quite small and easily concealed on a person's body or in a bag, Dr NicDaeid added.

"Ten pounds weight is a fairly small size," she said. "It would be consistent with something like a pipe bomb-type device. In cases like this, an explosive device can fragment considerably. You are looking for what sort of device it was and what sort of explosives and chemical residues."

The vital role that forensic science can play in a terrorist investigation was shown in the Madrid train bombings on 11 March 2004, when 191 people were killed on commuter trains. Spanish police investigating 10 bombs that exploded on four packed trains secured a crucial early breakthrough when they discovered three more devices that had failed to go off.

The bombs were fitted to mobile phones, and the alarms were used as timers. Police arrested their first suspect, a Moroccan man who ran a small phone business, almost immediately.

One of the most important forensic inquiries being made by the Metropolitan Police will be to establish whether any of the bombers died in the attacks. It looks increasingly likely that the terrorist responsible for the bus blast perished in the attack, either because he was a suicide bomber, or the device detonated by accident.

If the terrorist is among the dead they may be able to identify him via his dental records or DNA. This, in turn, will help the police track the other bombers. Forensic scientists will be able to establish whether the bomber was among the dead by looking at the blast pattern and injuries on the dead. These can be compared with known suicide bomber case, such as the attacks in the Middle East.

Sir Ian Blair, the Commissioner of the Metropolitan Police, said there was so far "absolutely nothing" to suggest the attack on the bus had been the work of a suicide bomber but added: "We cannot rule it out, it may have been, but it [the device] may have been left on a seat or it may have gone off in transit."

But before the bulk of the forensic science work can properly get started on the scenes of the four attacks the remaining bodies of the dead must be removed from the wrecked carriage on the Piccadilly line. An unknown number of bodies remain inside the carriage, which is about 500 yards from Russell Square station inside the deep Tube tunnel. Fumes, vermin, the risk of asbestos, stifling heat and the threat of the tunnel and the carriage collapsing have all seriously hampered the recovery operation and made life near-impossible for emergency workers.

It looks increasingly likely that the bomb-damaged carriage will have to be dismantled in the tunnel and taken up to the surface piece by piece.

The other major forensic task is to examine all the hundreds of hours of video footage recorded along the routes of the Tube trains and bus to try to identify the terrorists. A senior police source said: "It's probably going to be the biggest CCTV recovery that we have had."

He added: "The forensic recovery from the [blast] scenes could save lives in the future. We have to work on the assumption that the people responsible are still out there and they could do it again."

The police have appealed to anyone with information about terror suspects or the bombings to contact the anti-terrorist hotline on 0800 789321.

Key clues the explosives experts will be searching for

"The first thing the forensic teams would have done is take hundreds of pictures of everything, including the bodies. By building up a detailed picture of the bomb site, it is possible to determine the force of the explosion.

The most important thing about investigating bombs is preserving evidence: first at the scene and then at the laboratory. Forensic teams would also look at how the windows were blown out and the metal of the carriages deformed. This could be compared to other explosions - if the seats were blown off their brackets, you know it took a certain amount of force to do this. From this you can determine the force and the type and quantity of explosives used. Plastic explosives would make sense - relatively stable and easy to handle.

They would also examine all items and fragments at the scene. If there were scorch marks on a bag, for example, they could say the temperature had risen to a certain degree to cause that; or if they found metal fragments that were not part of the train these could be clues about the type of bomb used. The teams will probably be looking for the remnants of a timer. If it was a suicide bomber all he or she would have needed was a push-button device to set it off. So if they find a timer it would strongly point to it not being a suicide bomber."

Hans Michels, professor of Safety Engineering at Imperial College London, is a founding member of the UK Explosion Liaison Group