Metropolitan police are looking for an unknown number of fugitives connected to four explosions in London Thursday and are hoping the bombing material used can lead them to the terrorists.

Two weeks to the day after terrorist attacks in London killed a total of 56 people, explosions struck three more Underground trains and a bus at lunchtime Thursday.

Metropolitan Police commissioner Sir Ian Blair (search) confirmed that three explosions occurred on subway trains near the Oval and Warren Street stations on the Northern Line and on a train near the Shepherd's Bush station on the Hammersmith and City line, and that a fourth explosion took place on a bus traveling in the Hackney neighborhood of east London.

But none of the presumed bombs appeared to have detonated properly, leaving only one person injured. Officials were hoping that the leftover explosives would provide a wealth of forensic evidence to help investigators hunt down whoever planted them.

"Clearly, the intention must have been to kill. You don't do this with any other intention, and I think the important point is that the intention of the terrorists has not been fulfilled," Sir Ian Blair during a press conference Thursday afternoon.

Police said two men were arrested. One was detained near Downing Street, site of the prime minister's residence. The other, later released without charge, was picked up near Tottenham Court Road, close to the Warren Street subway station where one attack took place.

Detectives are working on the basis that the bombs were not properly primed, sources told Sky News. That could explain eyewitness accounts of suspects fleeing the scenes of at least some of the blasts.

Police officials said it was too early to tell whether the failed bombings were the doings of an Al Qaeda or affiliated cell. They did say, however, that there was a "clear similarity" between Thursday's incidents and the July 7 bombings.

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Blair said the hospitals were ready to handle any injured patients and the explosion scenes were very tightly controlled.

"We do believe that this may represent ... a significant breakthrough in the sense that there is obviously forensic material at these scenes which may be very helpful to us, so I feel very positive about some of these developments," Blair said.

One Sky News correspondent reported that officials said the explosive device on the bus appeared to be made of TATP, the same highly unstable material used in the July 7 bombings. A large area around the bus had been cordoned off.

Police told people in the area of the bombs to get off their cell phones. The concern is that the phones, which work on radio frequencies, could set off potential leftover bombs.

An armed police unit at one point entered University College Hospital, near the Warren Street station. Sky News TV reported that police were searching for a black or Asian male, 6 feet 2 inches tall, wearing a blue shirt with wires protruding out a hole in the back. "Asian" in Britain refers to people with origins in the Indian subcontinent.

A Criminal Act

Prime Minister Tony Blair said Thursday that officials hoped that London would return to normal "as soon as possible."

"We can't minimize incidents such as this," Blair said during a news conference with visiting Australian Prime Minister John Howard, adding that such attacks were meant "to make people worried and frightened and taking responsibility off the shoulders of people who engage in these acts."

Blair said it was "important to respond by keeping to our normal lives."

"People have seen our country react to terrorist attacks that are meant to make people worried and scared and react, and people have reacted with great dignity. ... It's not going to change us … Our reaction's got to be the same" as it was after the July 7 attacks, Blair added.

Blair the police commissioner said "attempts" to set off explosives were made at the four scenes.

"This is an incredibly fast-moving scene, a very fast-moving set of events," he said. "The main message is now that London has gone past this point again, we're back to business."

Although Hammersmith, City, Victoria and Northern Underground lines were still closed, shuttles were running to aid commuters, Blair said.

Officials urged Londoners to report any suspicious activity to them and to send information to the Web site www.police.uk.

Blair also urged patience and restraint, in obvious reference to any retaliation against London's Muslim community that may ensue; Islamic extremists are believed responsible for the July 7 attacks and others around the world.

"No community should be smeared with responsibility for these matters. These are criminal acts and we are in pursuit of a set of criminals in relation to it," Blair said.

'Something Had Gone Wrong'

After the explosions, the Warren Street, Shepherd's Bush and Oval stations were evacuated. There were reports of a nail bomb that exploded and/or a backpack that exploded at Warren Street. No chemical agents were involved.

Some witnesses said they heard gunshots at the Warren Street stop but police believe those noises were actually detonators going off for bombs that didn't explode.

Witness Ivan McCracken told Sky News when he got off the train at Warren Street, he spoke to an Italian man who was comforting a woman after the evacuation.

"He said that a man was carrying a rucksack and the rucksack suddenly exploded. It was a minor explosion but enough to blow open the rucksack," McCracken said. "The man then made an exclamation as if something had gone wrong. At that point everyone rushed from the carriage."

Stagecoach, the company which operates the stricken No. 26 bus, said the driver heard a bang and went upstairs, where he found the windows blown out. The company said the bus was structurally intact and there were no reports of injuries.

Although not as serious, Thursday's incidents were hauntingly similar to the blasts two weeks ago, which involved explosions at three Underground stations simultaneously — quickly followed by a blast on a double-decker bus. Those bombings, during the morning rush hour, also occurred in the center of London, hitting the Underground railway from various directions. The four homicide bombers, along with 52 others, were killed in those attacks.

Thursday's incidents, however, were more geographically spread out.

The explosions came as Pakistani intelligence officials said authorities are seeking the former aide of a radical cleric in Britain in connection with the July 7 bombings.

The officials said British investigators asked Pakistani authorities to search for Haroon Rashid Aswat (search), who reportedly had been in close contact with the suicide bombers just before the July 7 attacks. Aswat, 31, was of Indian origin and may not be in Pakistan, according to two intelligence officials in Islamabad and one in Lahore.

FOX News' Amy Kellogg and The Associated Press contributed to this report.

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