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LONDON, England (CNN) -- Four men captured on closed-circuit television in a London subway station shortly before a series of deadly bombings rocked the city last week have become a focus of the massive police investigation.

Investigators are trying to determine if the four men may have died in the blasts on the capital's transit system, the worst attack Britain has seen in peacetime.

Police were led to the four men after the family of one reported him missing just hours after the bombings, which have so far killed at least 52 people.

On Tuesday, police searched six locations in West Yorkshire, where three of the men lived, and made one arrest. The home addresses of three of the men were among the locations searched, police said.

In Luton, a town about 30 miles (48 kilometers) north of London which houses one of the city's three major airports, police carried out a controlled explosion on a car parked at a train station that was believed to be linked to the bombings.

Before investigating the vehicle, police closed the train station and parking lot and cordoned off the surrounding area.

Police have confirmed the identity of one of the four suspects as Shahzad Tanveer, from Leeds, a city in West Yorkshire about 200 miles (320 kilometers) north of London. His personal belongings were found on a bus that was blown up in the attacks, police said.

Tanveer's neighbor told the British television network ITN the 22-year-old had told him he had traveled to Afghanistan and Pakistan, spending a couple of months in Afghanistan and four months in Lahore, Pakistan.

Many of the people living in the same street as the Tanveer family are from Pakistan and the disputed Kashmir region. However, Muslims make up a relatively small percentage of the overall population in the area.

Tanveer, of Pakistani origin, was reported missing by his family within hours of the attacks, said Colin Cramphorn, chief constable of the West Yorkshire Police.

Deputy Assistant Commissioner Peter Clarke, head of Scotland Yard's anti-terror branch, said the family's report led investigators to the four men, three of whom were from West Yorkshire.

Authorities say the man reported as missing "was joined on his journey to London by three other men."

All four had arrived in London by train on the morning of the bombings and were seen on closed-circuit television just before 8:30 a.m. at King's Cross station, he said.

Did they die?

The first of the four blasts, at the height of rush hour Thursday, went off just before 9 a.m. on a train that had just left King's Cross. Two other subway carriages in central London were also bombed, and the fourth and last explosion destroyed a double-decker bus shortly before 10 a.m. At least 52 people were killed.

"The investigation, quite early, led us to have concerns about the movements and activities of four men," Clarke said.

"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."

The police found personal documents bearing the names of three of the four men near the train seats where the bombs exploded, Clarke said, and Tanveer's personal belongings were found on the bus blown up in the attacks.

"Very strong forensic and other evidence" led police to believe one of the West Yorkshire men died in the explosion on a train between the Aldgate and Liverpool Street stations, Clarke added

Arrest made

Authorities made one arrest Tuesday while executing six search warrants in West Yorkshire. The man will be brought to London for questioning, Clarke said, providing no further detail.

Five of the entries police made during their searches were consensual, while the sixth occurred at an unoccupied property, where police, aided by army troops, used a "controlled explosion" to gain entry, Cramphorn said.

"There is material in the premises that cannot be identified as harmless. Therefore, we treat it as potentially hazardous until we have satisfied ourselves that it is harmless," Cramphorn said.

"That's a complex and lengthy process and is likely to run on for several days, up to a week in some of the properties," he said.

Before the explosion, more than 500 people were evacuated from homes and businesses, including a mosque, police said. Alternate arrangements were made for Muslims to pray, police said.

London police have taken more than 700 witness statements and received more than 2,000 calls to the anti-terrorist hotline, said Andy Hayman, London police assistant commissioner and head of specialist operations.

Officers are also reviewing 2,500 tapes of closed-circuit television footage from across the capital, as scores of families await news of loved ones feared killed.

Officials have said they expect the number of dead to rise. Scotland Yard said Tuesday 11 victims have so far been positively identified.

Forensics experts have said it could take weeks to identify all the bodies recovered, many of which were mangled in Thursday's attacks.

Other developments

• U.S. officials lifted a ban preventing about 10,000 U.S. military personnel -- based at RAF Mildenhall and RAF Lakenheath, both in the eastern English county of Suffolk -- from going into the Greater London area. The instruction to U.S. forces had contradicted the message from politicians, including London's mayor, for people to return to the capital. (Full story)

• Britain's finance minister, Gordon Brown, vowed Tuesday that the European Union would unite to defeat terrorism and clamp down on the financing that feeds it. (Full story)

Police: Did 4 London bombers die?