"A slightly different picture is emerging around the timing of these bomb incidents," Deputy Assistant Commissioner Brian Paddick said on Saturday at a Metropolitan Police briefing on Thursday's bombings.
"All three bombs on the London Underground system actually exploded within seconds of each other, at 8.50 in the morning."
Sophisticated coordination is a hallmark of al-Qaida, the network blamed for the 11 September attacks on the United States and said by British officials to have possibly been behind the London blasts.
Police said the bombs were composed of "high explosive" - probably not home-made material.
The first bomb exploded at the Aldgate station in east London. Two more went off within seconds, they said. A fourth destroyed a bus near a subway entrance.
Forty-nine bodies have been recovered from the bombings, but some bodies remain trapped underground, police said.
Recovery crews were hampered by heat, dust and other "difficult conditions," Deputy Commissioner Andy Trotter of British Transport Police said.
In a BBC radio interview on Saturday, Prime Minister Tony Blair said investigators did not yet know who was behind the attacks, but hoped to have more information soon.
Blair: Investigators still do not
know who was behind the attacks
He said he was aware of a claim of responsibility posted on the Internet by a group calling itself The Secret Organisation of al-Qaida in Europe.
He said it was "reasonably obvious that it comes from that type of quarter", but not yet clear exactly which organisation was responsible.
A second claim appeared on a Web site on Saturday, this one signed Abu Hafs al-Masri Brigades, the group, whose name evokes the alias of Mohammed Atef, Osama bin Laden's top deputy who was killed in a US airstrike in Afghanistan in November 2001.
But experts say the group has no proven track record of attacks, and note it has claimed responsibility for events in which it was unlikely to have played any role, such as the 2003 blackouts in the United States and London that resulted from technical problems.
Investigators have yet to name any suspects, but British
newspapers on Saturday said police had asked European counterparts for information on Moroccan cleric Mohammed al Garbuzi, who lived in Britain for 16 years before vanishing from his north London home last year.
Aljazeera managed to reach al-Garbuzi in London on Saturday and he denied that he was in hiding or is being sought by the British authorities.
He added that his address is known to the British police.
Morroco has sentenced al-Garbuzi to 20 years in jail on charges that he was involved in plotting deadly bomb attacks in Casablanca in 2003.
Meanwhile, in the BBC interview, Blair also said it was crucial to address terrorism's underlying causes, which he identified as deprivation, lack of democracy and ongoing conflict in the Middle East and praised the calm way Londoners reacted to the bombings.
His countrymen, he said, "are simply not going to be terrorised by terror in this way."
The bombs that destroyed the three London Underground cars and a double-decker bus were each lighter than 4.5kg and could be carried in a backpack, police said on Friday.
Underground passengers around the capital remained wary on Saturday, a day after most of its lines reopened. Commuters remained steady but light.
Underground passengers remain
wary a day after lines opened
"Everyone's looking around a little bit more," said William Palmer, 23, a student.
At King's Cross station, near the site of the deadliest of the three subway bombings, Underground service was partially restored on Saturday.
Crews reached the cars near St Pancras and said they saw more bodies in the hot, rat-infested tunnels, but warned it could take days to retrieve them. Many of the tunnels are more than 30m beneath the surface.
"This is an enclosed tunnel and it is very difficult conditions and it's a bit dangerous," Transport Police Deputy Chief Constable Andy Trotter told reporters on Friday.
"It will take some time before everyone is removed from there."
Sir Ian Blair, commissioner of the Metropolitan Police, said he expected the toll to rise, but doubted it would reach triple digits.
More than 700 were injured in Thursday's attacks on London, Ian Blair said, adding that the injured included citizens of at least five countries in addition to Britain - Sierra Leone, Australia, Portugal, Poland and China.
Germany, Denmark and South Africa also reported injured citizens.