Three bombs containing sophisticated explosives hit the London Underground within less than a minute of each other, police said Saturday as a clearer picture emerged of the coordinated attacks last week that killed at least 49 people.
The bombs on the subway went off within a span of 50 seconds Thursday, suggesting detonation by synchronized timers rather than suicide bombers, police said, revising earlier accounts that the blasts occurred within a 26-minute span. An explosion tore through a double-decker bus nearly an hour later.
The explosions were so destructive that authorities haven't been able to identify a single body and were depending on fingerprints, dental records and DNA analysis, detectives said Saturday.
More 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.
Sir Ian Blair, commissioner of the Metropolitan Police, said he expected the toll to rise but doubted it would reach triple digits. Many of the Underground tunnels are more than 100 feet beneath the surface.
CBS News Correspondent John Roberts reports there's a glum acceptance among London's residents that state-of-the-art surveillance and security couldn't save lives.
"It's almost impossible to stop something like this, at least in my opinion," Mike Adams tells CBS News. "People walk around with backpacks all over the place.
"We're carrying one now. You can't check every single person - it's impossible."
Sophisticated coordination is a hallmark of al Qaeda, the terror network blamed for the Sept. 11 attacks on the United States and said by British officials to have possibly been behind the London blasts.
"A slightly different picture is emerging around the timing of these bomb incidents," Deputy Assistant Commission Brian Paddick said 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."
The first bomb exploded at the Aldgate station in east London. Two more went off within seconds, they said.
Police said the bombs were composed of "high explosive" — probably not homemade material. Investigators said Friday that the bombs were lighter than 10 pounds each and could be carried in a backpack.
A fourth destroyed a bus near a subway entrance, killing 13 people, police said. The attacks came as President Bush and other G-8 leaders were holding a summit in Scotland and a day after London was named the host city for the 2012 Olympics.
CBS News Correspondent Jim Stewart reports
Stewart reports investigators now suspect all the bombers used what they called the "step-on, step-off" delivery system of dropping off a bag and quickly leaving.
Forty-nine bodies have been recovered from the bombings but were so mangled that detectives have not been able to identify a single body.
More than 700 people from several countries, including the United States, were wounded.
In a British Broadcasting Corp. radio interview Saturday, Prime Minister Tony Blair said investigators did not yet know who was behind the attacks but hoped to have more information soon.
He said he was aware of a claim of responsibility posted on the Internet by a group calling itself The Secret Organization of al Qaeda in Europe. He said it was "reasonably obvious that it comes from that type of quarter" but not yet clear exactly which organization was responsible.
Little was known about the group, but a Web statement in the same name claimed responsibility for the last major terror attack in Europe: the bombs on commuter trains in Madrid in March 2004 that killed 191 people.
A second claim appeared on a Web site 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 U.S. 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.
Subway passengers around the capital remained wary on Saturday, a day after most of its lines reopened. Rider volume remained steady but light.
"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. Flowers and sympathy cards piled up outside honoring the dead — at least 21 were killed on the train bombed between King's Cross and Russell Square stations.
"Madrid is with London," read one card. Another said: "Everyone has gone to the best place, which is heaven."
In the interview, the prime minister 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 terrorized by terror in this way."
The queen expressed her admiration for all the Londoners who "are calmly determined to resume their normal lives."
"Sadly we in Britain have been all too familiar with acts of terror and members of my generation, especially at this end of London, know that we have been here before," she said during a visit to the Royal London Hospital, referring to the Nazi blitz of World War II.