IN A barbed wire-bordered compound in Kent, forensics experts with infra-red cameras are combing and cataloguing every millimetre of a red double-decker bus, while teams of scientists tease apart volatile packs of explosives, using the tools of chemistry to rewrite the bombmaker’s signature.

In a dingy 1970s office block in Lambeth, police are hunched over screens, watching hour after hour of CCTV footage in pursuit of the fugitives.

And inside Scotland Yard and MI5 headquarters, police chiefs and intelligence officers are collating and analysing the unexpected windfall of hard scientific evidence. Part of that windfall went on display yesterday in the shape of four grainy images, proof of the speed at which technology is closing in on the terrorists.

There was something close to elation in the voice of Sir Ian Blair, the Metropolitan Police Commissioner, when he declared that bombs had been recovered almost intact: “We believe this may represent a significant breakthrough.”

The unexploded bombs are a treasure trove, an own-goal by the bombers that should yield vital clues about the terrorists, their bombmaker and their methods.

The contrast with the evidence available after the first wave of bombings is stark: after July 7, investigators were left to piece together tiny shards of exploded material in the wreckage, swabbing minute particles of DNA left by victims and terrorists, combing CCTV tapes for the faces of four unknowns.

This time they have several entire bombs, rucksacks with fingerprints, witnesses and film of the bombers not only before the attack but also as they fled. By yesterday morning they also had a dead body, of the man shot at Stockwell station, and a face to compare with the evidence already accumulated.

Scores of police have been diverted from normal duties, and at least 1,000 officers of the Metropolitan Police are at work on the biggest forensics investigation in British history, a vast and hugely complex jigsaw of science, technology and old-fashioned police work.

A prime focus is the No 26 bus, which was towed away from the Hackney Road yesterday by forensic officers in white suits and masks. It is believed to have been taken to Fort Halstead, the top-secret armaments research establishment on the North Downs near Sevenoaks. Fort Halstead, one of the most-crucial but least-known arms of the Defence Ministry, is where experts pieced together the wreckage of the Pan Am airliner that exploded over Lockerbie in 1988.

Shortly before 10pm on Thursday, a two-man bomb disposal unit boarded the bus and removed the unexploded bomb and the rucksack that had contained it. The bomb was transferred to a steel canister. Police then photographed the bus inside and out after a fingertip search.

Some of the analysis is likely to take place at the laboratories in Lambeth Road, an unremarkable tower block that is home to the Government’s Forensic Science Service.

Every bomb carries the chemical “signature” of its maker, based on the chemical admixture, and the type of detonators, casings, wire and tape. Information on the signatures is shared internationally by anti-terrorist forces. Bombers also tend to construct their weapons according to where they learnt the technical skill, providing further clues. The first task will be to establish the similarities, if any, between the explosive mixture used on July 7, the bombs found in the car at Luton and the explosives recovered in Leeds, and this week’s unexploded material.

The earlier bombs are believed to have been made of acetone peroxide — triacetone triperoxide or TATP — an ingredient popular with Middle Eastern terrorists. TATP can be made from common products including sulphuric acid, hydrogen peroxide and acetone. If the precise chemical make-up is established, then steps can be taken to start detecting it on the transport system.

Three of the four devices were thought to be of a similar size and weight to those used on July 7. The fourth was smaller and contained in a plastic box. On three of the devices, the detonators went off but the bomb failed to explode, and on the fourth the detonator apparently failed.

The Lambeth Road labs will also be testing for DNA, fibres and fingerprints at the scenes. A single drop of sweat on a piece of plastic can be enough to obtain a DNA match.

The rucksacks will also be closely examined. The bag on the bus bore the logo “Fitness First”. The gym company of that name said that about 200,000 bags are given away annually to customers.

In the past, forensic science has provided the key in terrorist cases. After the Madrid bombings a thumbprint on a telephone Sim card led police to their first suspects.

Another key strand to the investigation is visual: yesterday officers continued to gather CCTV footage from cameras in the Tube and on the route of the No 26. About 25,000 tapes are expected to be gathered in the course of the investigation, with police spreading outward from the sites of the attacks to establish a “narrative” of the bombers’ movements.

Police were collating hundreds of witness accounts, and sifting through thousands of calls yesterday.

They worked in pairs on the footage, trying to match faces to the accounts of witnesses, some of whom tried to tackle the bombers as they fled. Nasa technology is used to enhance faint or grainy images. The Lambeth Road viewing suites were supplemented by those used by the vice squad and commercial suites.

London has invaluable expertise in such investigations, even if the scale has changed. Science and technology have already transformed the investigation. Twenty-four hours after the bombings on July 7, the dead terrorists remained faceless; barely 24 hours since the second attack, there are four faces and at least three terrorists on the run.

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