Spanish experts who helped tracked down suspects in the March 11, 2004, Madrid train blasts have flown in to join the British investigative team, along with Europol specialists.
Investigators say they have already established that the bombers used a high explosive they will not disclose further details and that each device was lighter than 10lb (4.5kg).
The three blasts on the underground rail system went off within 50 seconds of each other, suggesting the use of a timer rather than a hand-set explosion by a suicide attacker.
The fourth bomb, which ripped the roof off the bus, appears to have been planted in a bag.
Even as weeping mourners emerged from a service at the St Pancras Church near the site of a blast that peeled the roof off the Number 30 double-decker bus, a line of seven forensic experts could be seen at work.
Dressed in blue overalls, foam pads tied to their knees, they crept forward with eyes glued to the ground, dipping every few steps to pick up the tiniest scraps that might furnish a clue. Behind the detectives, sheets of white tarpaulin hemmed in the zone around the bus explosion, the only one of four crime scenes to be visible.
British police were poring over everything from the entire top of the bus carted away on Saturday for deeper investigation and the tiniest splinters that will help them recreate the explosions.
Thousands of hours of closed circuit video recordings are also under examination.
With the al-Qaida network the focus of the country's biggest manhunt, the painstaking search has been given added urgency by the suspicion that Osama bin Laden's network may plan a second strike.
The direction, force and type of blast revealed by the deformation of the surrounding objects can give pivotal clues, said an explosives expert at London's Imperial College, Hans Michels.
"The next thing to do would be to carefully pick up all the bits and pieces, take many, many photographs and store items in envelopes and plastic bags, sending samples off for analysis," he said.
Scientists would try to work out how much energy was released in the blast and what type of explosive was used.
Family and friends of more than 30 missing people have made anguished visits to a special support centre set up in the wake of the London terror attacks. Police have also sent specially trained liaison officers to the homes of 57 families.
As part of the support operation for families, samples of fingerprints and DNA including from toothbrushes and hair brushes will be taken to help in the identification process.
The 24-hour family assistance centre at the Queen Mother leisure centre in Victoria, central London, is being staffed by 100 police officers and staff, supported by workers from the British Red Cross, Salvation Army, Victim Support and several local authorities.
Liz Page of the British Red Cross said: "Our people are trained to hear some very difficult things."
Michael O'Connor, director of children and family services at Westminster Council, said he was not aware of anyone reporting children missing.