London commuters reluctantly descended into the Underground Friday morning, but buses and subways carried fewer riders than normal the day after
Police raised the death toll to more than 50 and said each of the bombs contained less than 10 pounds of explosives.
At least 700 people were wounded, with 100 still in the hospital, including 22 in serious or critical condition with burns, amputations and fractures.
Sir Ian Blair, commissioner of the Metropolitan Police, said work continued to establish the precise number of dead from the explosions on three subway trains and a double-decker bus.
Blair said there was no evidence that the attacks had been carried out by suicide bombers.
"We have absolutely nothing to suggest this was a suicide bombing attack although nothing at this stage to rule that out," he told a news conference.
Assistant Police Commissioner Andy Hayman said that officials still hadn't gotten near the subway cars of the Russell Square station, fearing that the tunnel is unsafe. Twenty-one dead were confirmed in that blast.
He said officials believe the bombs were placed on the floors of the three subway cars that were hit. He said the initial investigation suggests that each bomb had less than 10 pounds of explosives.
He appealed for patience as the investigation proceeds. "Our people are working under the most extreme circumstances."
London's mass transit system reopened Friday, though some commuters, admitting they were afraid, opted for a taxi. Normally packed double-decker buses carried just a handful of passengers, and many Underground stations were less congested than normal. But others said they had little choice but to board the subway.
"I was scared, but what can you do?" said Raj Varatharaj, 32, emerging from an Underground station. "This is the fastest way for me to get to work. You just have to carry on."
Michael Clarke, a professor of defense studies at Kings College in London, told CBS News' The Early Show that "London, like New York, gets back to normal fairly quickly. London has not been surprised by this, they've not been shocked by it. They're very saddened by it. But they're very fatalistic. They expected something like this to happen."
Stewart says 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.
Based on evidence recovered from the rubble, investigators believe some of the bombs were on timers, a U.S. law enforcement official said. The official would not further describe the evidence.
Investigators doubt that cell phones — used in the Madrid train attacks a year ago — were used to detonate the bombs in the Underground because the phones often don't work in the system's tunnels, the official said. The official spoke on condition of anonymity because the investigation is ongoing.
and around the world.
"I want Americans to know that our transit system is safe. I want them to be vigilant, but I want them to go about their business and continue to pursue their daily life," Homeland Security Secretary Michael Chertoff told CBS News' The Early Show.
Much of Europe was also on alert, and Italy's airports have raised alert levels to a maximum.
A group calling itself the "Secret Group of al Qaeda's Jihad in Europe" has claimed it was behind the attacks, but that claim has not been verified. In a posting on a Web site, the group said the bombings are punishment for Britain's involvement in the war in Iraq and invasion of Afghanistan.
It threatened to attack Italy and Denmark for their support of the U.S.-led coalitions in both countries, too.
A senior U.S. counterterrorism official said the posting is considered to be a "potentially very credible" claim, in part because it appeared soon after the attacks.