Following a morning of dramatic raids, Britain went to bed Tuesday with the possibility the people who bombed London's subways last week were domestic suicide bombers.

If they are, the four men -- ranging in age from 18 to 30 -- would be the first suicide bombers in British history.

"The investigation led us to have concerns about the movements and activities of four men," Peter Clark, head of the Metropolitan Police anti-terrorism squad, told reporters in London.

Three of the four men lived in Leeds, West Yorkshire -- a city of 715,000 about 300 kilometres north of London.

The three men travelled by train from Leeds to Luton, about 50 km north of London. From there, a fourth man joined them. They all travelled together by commuter train into London.

Security cameras captured images of them at King's Cross station at about 8:30 a.m. local time. They were wearing backpacks and chatting among themselves before setting off in different directions.

About 20 minutes later, three bombs exploded on three different subway trains. Those explosions occurred within a minute of each other. Just over one hour later, a fourth blast destroyed a double-decker bus.

The confirmed death toll is now 52 -- and that may include the bombers themselves.

"We have since found personal documents bearing the names of three of those four men close to the seats of three of the explosions," Clarke said.

CTV's Tom Kennedy said there have been documented cases of bombers leaving personal effects at the scenes of explosions to give the impression they died.

The authorities are currently involved in efforts to identify those killed in the blasts. The process won't be easy, as some people were literally blown to bits. DNA or dental records are the only options for identifying some.

Clarke said there was "strong forensic and other evidence" that one bomber definitely died on the scene of an attack, but they were still awaiting confirmation from the coroner.

"One of the men who set out from West Yorkshire was reported missing by his family to the Central Casualty Bureau," he added.

Some residents say a 22-year-old man who lived in Leeds hasn't been seen since the day of the bombings.

Passengers on the No. 30 bus reported seeing an agitated man in his 20s fiddling with something inside his bag.

Who are they?

The police haven't yet answered the question of who the men are, but the Guardian newspaper's website reported three of the men were Pakistani in origin. The Associated Press quoted a Leeds town councillor as saying three of the presumed bombers were British citizens of Pakistani ancestry.

The Guardian also reported that security forces described all four suspects as "cleanskins," meaning they had no documented involvement with terror groups or criminal records.

There were four raids in Leeds proper Tuesday morning, and two in Dewsbury to the south of the city.

The Leeds neighborhoods have significant Muslim populations, both Arab and South Asian. Public signs in storefronts are often in Arabic and English.

Police arrested one suspect, reportedly a relative of one of the presumed bombers.

The military literally blew the door down on one unoccupied row house there so police could enter and search for explosives and computers. Authorities reportedly found some additional explosives in the raids.

A spokesman for the Hyde Park Mosque in Leeds told AP that the Muslim community is surprised by police claims the bombers came from there.

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

The BBC reported police found explosives in a car parked at a rail station in Luton, about 50 km north of London.

Metropolitan Police officers from London examined the car, removed the explosives and then detonated them in a controlled explosion.

Police removed the car, believed to be linked to the July 7 attacks, for forensic examination. A second car was also impounded.

Kennedy said if the killers are dead, it is still vital to know if they acted alone or had a support network. For example, did they get help in obtaining explosives?

"But I suppose the real urgency is still, are there others like them waiting to strike?" he said.

With a report from CTV's Tom Kennedy