The three bombs on London underground trains on Thursday exploded almost simultaneously, police have said.
Scotland Yard said the attacks took place within 50 seconds of each other. It was previously thought they had taken place over a longer time period.
Police have also warned the recovery of victims could take "days more".
There have been 49 confirmed fatalities in the bomb attacks, while concerns remain for a further 25 missing people. At least 700 were injured.
A two-minute silence is to be held in memory of those killed at 1200 BST on Thursday, the government said.
Culture Secretary Tessa Jowell told BBC News ministers were planning a special site in London where "families very particularly" could come together to observe the silence.
Anxious family and friends are continuing their search for loved ones who have not been heard from since the bombings.
Technical data from London Underground disproved the earlier wider time range between explosions, Scotland Yard Deputy Assistant Commissioner Brian Paddick said.
Police have released the first pictures of the train at Aldgate
There is still no certainty about the number of people whose bodies remain trapped in wrecked train carriages below King's Cross.
The recovery operation would be a "slow, methodical, meticulous process" in very difficult circumstances, said Deputy Chief Constable Andy Trotter.
It would take "as long it takes", he said.
There are still bodies at the two other Tube bomb sites - at Aldgate station and Edgware Road - but it is thought the King's Cross site is the only one where bodies remain on the train.
Forensic experts continue to gather evidence from all the sites as well as at Tavistock Square, the scene of a bus bombing an hour after the Tube attacks.
Mr Trotter said there would not be "airline security" on the Tube, train or buses.
"If life in London is to continue then people are going to get on and off public transport without going through search regimes," he said.
So far no victims have been formally identified - and police warn that the process, due to begin on Saturday, could take weeks to complete.
A 24-hour reception centre has been opened at the Queen Mother Sports Centre in Victoria, to help the families of people not seen since the explosions.
The police say the Tube explosions took place at 8.50am - and the synchronisation could suggest bombs used in the attack were triggered by timing devices.
High explosives were used in the attacks and were not home-made, say the police.
Mr Paddick denied reports investigators were looking "for any specific individual".
Prime Minister Tony Blair warned security and surveillance would not be enough to stop such attacks - and there had to be an ideological struggle in which terrorism is "pulled up by the roots".
Speaking on BBC Radio 4's Today programme Mr Blair praised the "inner resilience" of Londoners - as the capital's transport system began to return to a close-to-normal service. There are now services running on sections of all Tube lines.
Prince William signed a book of condolence in Auckland, New Zealand.
Terrorism experts from Spain who investigated the Madrid bombings are supporting the inquiry.
A claim for the attacks has been made in the name of al-Qaeda by a group calling itself the Abu Hafs al-Masri brigade.
But the BBC's security correspondent Gordon Corera has urged caution over the credibility of the claim.
Forensic teams working in Tube tunnels and at the other scenes of the blasts are taking swabs to try to determine the type of explosives used.
The roof of the number 30 bus, which was ripped off in the blast at Tavistock Square, has been removed from the scene for forensic examination.
Police are also involved in one of the UK's biggest searches of CCTV footage to see if there are any clues as to the identity of the bombers.
BBC security correspondent Frank Gardner said there were a number of key questions which investigators were analysing.
"One of the most important is were the bombers home-grown British terrorists or was this a hit team that came in from abroad?" he said.
One possibility being investigated was that the bomb maker was an expert who came and instructed the bombers.
Another area was whether they were "linked directly" to what was left of the core of al-Qaeda in Afghanistan and Pakistan, or acting alone.
Search for missing
An emergency call centre in London has taken more than 120,000 calls from the public.
Anti-terrorist hotline: 0800 789 321
Missing relatives: 0870 156 6344
Those looking for missing people have been contacting hospitals, as well as taking photos and posters to the four blast sites.
Scotland Yard confirmed seven people died in the Liverpool Street explosion, another seven at Edgware Road, a further 13 in the Tavistock Square bus blast and at least 21 at the King's Cross blast. A 49th person died in hospital later.
Of the 700 people hurt, about 69 are being treated in hospital and 15 remain in a critical condition.
A group of Muslims held a vigil outside St Mary's Hospital to show their solidarity for the victims of the attacks.
Between Aldgate and Liverpool Street tube stations
Between King's Cross and Russell Square tube stations
At Edgware Road tube station
On bus at Tavistock Square