Four small explosions shut down the city's public transit system at lunchtime Thursday and sent panicked passengers fleeing for safety in an echo of the deadly suicide strikes of two weeks ago. Police reported one injury.
Just as on July 7, men carrying bombs concealed in backpacks hit three subway trains and a double-decker bus in quick succession. Initial investigation suggested that explosives in the bags only partially detonated, officials said, preventing carnage on a scale of the previous attacks, which killed at least 56 people, including the attackers.
Witnesses described sharp bangs that created acrid clouds of thick white smoke. Attackers were seen fleeing on foot. In at least one station, passengers chased after a man they believed to be the culprit, but he got away.
Investigators said they were pursuing several unidentified suspects, but gave no details. Two men were arrested, but were quickly released, officials said. Authorities acknowledged that, as with the previous bombings, they had had no intelligence warning of the attacks, though the city is on a high state of alert.
Backpacks left behind by the men, numerous witness accounts and footage from dozens of security cameras gave authorities a huge trove of evidence to sift through. But police said they did not know whether the attackers were linked to the four young British Muslims suspected of carrying out the July 7 bombings or were amateur copycats who failed to detonate lethal amounts of explosives.
"Clearly the intention must have been to kill," Ian Blair, commissioner of the Metropolitan Police, told reporters.
"There is a resonance here, isn't there?" said Blair, referring to the July 7 attacks. "Whether or not this is directly connected in the sense of being carried out by the same group of people, however loosely knit -- that is going to take just a little bit longer" to determine.
Officials refused to describe the explosives used, although they said they were investigating whether the materials were similar to those of the July 7 attacks.
One British source said Londoners had been lucky because some of the explosives apparently failed to detonate. Had they all gone off, the source said, they could have caused casualties and damage similar to those in the original blasts.
The attacks badly rattled a city where people had been starting to regain confidence after the past bombings. Transit systems were shut down in response for much of the day, forcing thousands of workers to make their way home on foot.
The first detonation occurred just before 12:25 p.m. on a subway car on the Hammersmith and City Line that was passing through an aboveground station at Shepherd's Bush in west London. Passengers told reporters that a muscular-looking black man ran from the platform, lowered himself down a wall and bolted through backyards.
Five minutes later, a second detonation occurred on the Northern Line at the Oval station, south of the Thames River. Witnesses described it as a sharp popping sound like champagne corks and said it came from a backpack that an Asian man who looked about 19 had dumped inside a train.
Passengers pulled the emergency cord, opened the doors and fled. Some said they could see the still-intact backpack inside the car. Three men -- including one selling flowers just outside the station -- ran after the suspect, but he dashed up an escalator and escaped.
"People were trying to drop him, to rugby-tackle him," said Paul Martin, 32, who was on his way to a cricket match at the famed Oval grounds when the incident occurred. "There was a general melee."
Hugues Caillat, a French visitor, was buying a ticket in the station concourse when the suspect ran by. "He was running, and at the same time, people were running after him," Caillat said. "The guy said something like, 'What's wrong with these people?' He was a skinny Asian guy with a little beard."
About 15 minutes later, a third explosion was reported on the Victoria Line near the Warren Street station to the north. Once again, train passengers pulled the emergency cord and stampeded from a car filling with smoke, some of them screaming in fright. A man ran up the stairs and tried to fade into the crowd.
Witnesses said he headed off in the direction of nearby University College Hospital. Police sent an e-mail to some of the hospital's staff members asking them to be on the lookout for a black or Asian man, about 6 feet 2 inches tall, wearing a blue shirt or jacket with wires protruding from it.
Armed bands of police stalked the hallways, witnesses said, and ordered staff members to remain inside their offices. They eventually arrested a man whom they held at gunpoint for nearly two hours before taking him away.
"I've never seen so many police in my life," a staff member who did not identify herself told reporters, "and with guns as well."
Just as on July 7, the last attack occurred on a bus -- in this case on the No. 26 on Hackney Road in the Bethnal Green section of East London about 1:30 p.m. Passengers told reporters that they heard a small detonation inside a backpack at the rear of the upper deck -- the same location as the bomb that killed 14 people July 7. This time, however, the detonation caused only minor glass breakage.
The bus driver took a look at the backpack, which lay split open, and ordered everyone off the vehicle. Police then sealed off the area and called in a bomb disposal unit.
As happened two weeks ago, rescue workers and police initially hesitated to approach the bomb scenes for fear they were being lured into booby-trapped vehicles. They eventually examined each site and determined that no chemical or biological materials were present.
Still, the city throbbed with anxiety. Outside the security gates leading to Prime Minister Tony Blair's Downing Street offices, police wielding automatic weapons ordered an Asian-looking man carrying a backpack to halt, lie down on the sidewalk with his arms extended and then remove the bag while guards trained their weapons at his head. He was then hauled away. Office workers at Labor Party headquarters a few blocks away were ordered out of their building because of a bomb scare.
Both incidents turned out to be false alarms. Later, officials said both the man taken outside Downing Street and the man arrested at the hospital were released without charge.
Blair was meeting with Australian Prime Minister John Howard at the time of the explosions. At a joint news conference afterward, Blair praised Londoners for remaining calm.
"We know why these things are done -- they're done to scare people and make them worried," Blair told reporters. Although these incidents should not be minimized, he said, "what's important is that people do stay calm and react in the way they've reacted so far."
"To react in any other way," he added, "is to engage in the game they want us to engage in."
The entire London Underground was brought to a halt on "amber alert" for a brief time, but most of it was reopened before evening rush hour. Andy Trotter, a senior official with the London Transport Police, said his force was adding sniffer dogs, foot patrols and searches of suspected bags and packages, but he acknowledged that there was no way to guarantee safety on the transit system.
"This is a major worry and a major concern, and no one can pretend any different," he told reporters.
Some analysts said the fact that the attackers had failed to kill people suggested that they were amateurs lacking the skills of those behind the July 7 attacks, who investigators believe may have had help from al Qaeda operatives in Pakistan.
"It's too soon to be sure, but I'd say this group might turn out to have links to our friends from two weeks ago but not driven by the same plotters," said Michael Clarke, a terrorism expert at King's College London. "The guys behind July 7 were pretty good. And the failure of today's people to kill a lot of people is the sort of mistake they wouldn't have made."
Still, Clarke said, the attacks had proved successful in shaking the city's confidence by reminding Londoners how vulnerable they remain. "The anecdotal evidence of panic at Warren Street suggests people are far less sanguine than two weeks ago," he said. "And that's exactly what the terrorist wants. He wants to create a sense it can happen again at a time and place of his choosing."
"One is not enough," added Clarke, "but two is a pattern and three is a campaign."
Special correspondent Audrey Gillan in London and staff writer Robin Wright in Washington contributed to this report.