New evidence suggests four bombers blew themselves up on the London transportation system last week, killing at least 52 in what could be the first homicide attacks in Western Europe, officials said Tuesday.

The latest information indicates that three of the bombers were of Pakistani origin and that all of them were caught on film by London transit surveillance cameras just 20 minutes before the vicious assault, officials said. Two of the attackers were reportedly thought to be quite young, including a teenager and a 22-year-old.

As part of the probe, British police raided six homes in Leeds Tuesday, searching for computer files and explosives in the city in northern England. They arrested a relative of one of the suspected bombers, according to the British news agency Press Association.

The BBC also reported that explosives were found in a car at a rail station in Luton (search), 30 miles north of London. Police said earlier they carried out a controlled explosion on a car that was parked at the station and believed linked to the attacks. Metropolitan Police officers from London examining the car carried out the controlled explosion, Bedfordshire police said.

Police did not identify the four suspects believed to have been killed, but one bomber was thought to be Shahzad Tanweer (search), a 22-year-old cricket-loving sports science graduate, and another was a teenager, Press Association reported.

A town councilor told The Associated Press that at least three of the suspected bombers were British citizens of Pakistani ancestry.

Press Association said the men had driven a rental car to Luton, 30 miles north of London, and then boarded a commuter train to London's King's Cross station (search).

Closed-circuit TV video showed all four men arriving at King's Cross by 8:30 a.m. on Thursday, the day of the attacks, Peter Clarke, head of the Metropolitan Police anti-terrorist branch, told a Scotland Yard news conference.

Two militant Islamic groups have claimed responsibility for the attacks on three subway trains and on a bus. Police had previously indicated there was no evidence of homicide bombings, suggesting instead that timers were used.

Although police stopped short of calling them homicide attacks, Clarke said "strong forensic and other evidence" suggests one of the suspects was killed in a subway bombing and property belonging to the three others was found at the location of the other blasts.

"The investigation quite early led us to have concerns about the movements and activities of four men, three of whom came from the West Yorkshire area. We are trying to establish their movements in the run-up to last week's attacks, and specifically to establish if they all died in the explosions," Clarke said.

But Sky News, a sister network of FOX News, reported Tuesday that all four men — each armed with a separate bomb — perished in the blasts. The network quoted police sources.

The West Yorkshire region includes Leeds, and the homes of the three suspects from the city were among the six that were searched Tuesday.

Acting on six warrants, British soldiers blasted their way into an unoccupied, modest Leeds row house. Streets were cordoned off and about 500 people were evacuated. Hours earlier, police searched five homes elsewhere in the city.

Later Tuesday, police said a security alert was issued at Britain's House of Commons, but didn't say why. No evacuation had been ordered.

Mohammed Iqbal, a town councilor who represents the City-on-Hunslet section of Leeds, told AP that all of the homes raided in Leeds belong to "British citizens of Pakistani origin."

Three of the homes were in the neighborhood he represents, Iqbal said in a phone call with AP's office in Islamabad, the Pakistani capital. He said he had just met with police about the investigation.

"This is not good for Muslims," Iqbal said. "We have businesses here. There will be a backlash."

Several officials, including Foreign Minister Jack Straw, have said the attacks bore the "hallmark" of Al Qaeda, and one of the questions investigators are presumably trying to answer is whether the four suspects had outside help in planning the attacks.

Jeremy Shapiro, director of research at the center on the United States and Europe at the Brookings Institution, said Europeans had been involved in homicide attacks in the Mideast, but he knew of no successful homicide bombings in Western Europe previously.

Pakistan's government has been a key ally in the War on Terror, hunting down hundreds of Al Qaeda suspects and turning them over to the United States. But as in most of the Islamic world, its citizens have been deeply opposed to British and American military action in Afghanistan and Iraq.

A senior security official, who has viewed the closed-circuit TV footage, told Press Association that the men were talking casually as they carried their bombs in backpacks at the King's Cross station.

Clarke said police had strong evidence that the man believed to have carried a bomb onto the subway train that exploded between the Aldgate and Liverpool Street stations died in the blast, and they were awaiting confirmation from the coroner.

One of the suspects had been reported missing by his family at 10 p.m. Thursday, and some of his property was found on the double-decker bus in which 13 died, Clarke said.

"We have now been able to establish that he was joined on his journey to London by three other men," he said.

Some witness accounts suggested the bus bomber may have blundered, blowing up the wrong target and accidentally killing himself. A witness who got off the crowded bus just before it exploded told AP he saw an agitated man in his 20s fiddling anxiously with something in his bag.

"This young guy kept diving into this bag or whatever he had in front of his feet, and it was like he was taking a couple of grapes off a bunch of grapes, both hands were in the bag," said Richard Jones, 61, of Bracknell, west of London.

"He must have done that at least every minute if not every 30 seconds," he said.

One theory suggested the attacker may have intended to leave his bomb on the subway but was unable to board because his coconspirators had already shut the system down.

Investigators also found personal documents bearing the names of two of the other men near seats on the Aldgate and Edgware lines. Police did not identify the men.

Leeds, about 185 miles north of London, has a population of about 715,000. About 15 percent of the residents are Muslim, and many come from a tight-knit Pakistani community, mostly from Mirpur, south of Islamabad in Pakistani-controlled Kashmir. Other pockets of the community are mostly Arab, coming from a variety of countries including Syria and Saudi Arabia.

The military, including a bomb squad, carried out the controlled explosion at the row house at 11:30 a.m. so detectives could enter the home in Burley, a neighborhood where public signs in storefronts and even a Church of England community center are printed in English and Arabic.

No one was in the house at the time of the raid, police Inspector Miles Himsworth said. Detectives were scouring it for explosives and other items, possibly computers, he said.

Khalid Muneer, 28, a spokesman for the Hyde Park Mosque in Leeds, said the community was surprised by the raids and police claims that the bombers may have come from there.

"That connection would surprise us all, even shock the whole community. We still think it's too early to say," he told AP, adding that Muslims in the area were not opposed to Britain.

"I've seen no calls in this area for jihad against British or American forces. You will not get that sentiment expressed around this mosque."

Cordons kept bystanders about away from the house in Burley and police helped arrange prayers scheduled at a nearby mosque to be moved to other mosques nearby, Himsworth said.

Police said their painstaking investigation was moving ahead, and warned that the death toll would continue to rise. Fifty-six people remained hospitalized Tuesday.

Forensics experts have said it could take days or weeks to identify the bodies, many of which were blown apart and would have to be identified through dental records or DNA analysis. Investigators said late Tuesday that 11 bodies have been identified.

The Associated Press contributed to this report.