Security experts said the four, the first suicide bombers to strike western Europe, would have received training and direction from a more senior militant for their attacks, which killed at least 52 people.

That raised the prospect that a master bomb-maker was still at large and prompted questions about whether future suicide bombers had already been armed and instructed.

The bombers' background became the subject of a high-profile spat between London and Paris, with Home Secretary (Interior Minister) Charles Clarke denying comments by his French counterpart that British police had arrested some of the suspects in the past.

"It is completely and utterly untrue. I am absolutely staggered he should make that assertion," Clarke told Sky TV.

A British police source said it was possible some of the men might have come to the attention of police in the course of normal criminal investigations.

The BBC said police were hunting a fifth man, connected to the attackers, but not one of them. The report could not be confirmed by a London police spokeswoman.

"BRITISH BOMBERS"

Police have said four bombers blew up three underground trains and a double-decker bus last Thursday in an attack that officials say bore the hallmark of the al Qaeda network.

At least three of them came from the Leeds area in northern England, police said. The fourth was believed to be from the same area and a friend of the other three, Sky TV reported, citing a senior security source.

The three men identified by neighbors as the suspects were aged between 19 and 30, members of the sizeable ethnic Pakistani community around the city.

"They Were All British Bombers," declared the front page of the Daily Express newspaper, reflecting widespread shock, above a picture of the birth certificate of suspect Shehzad Tanweer.

Friends and family said the 22-year-old Tanweer was a cricket-loving sports science graduate with no interest in politics.

"I cannot believe it," his uncle, Bashir Ahmed, told reporters. "He wasn't into politics at all, so what drove him to do it? It can't be him, it must be something else behind him."

Irshad Hussain, who has known the Tanweer family for 25 years, said he believed the four had been tricked.

"I believe there was a mastermind behind this who had to kill the four boys because otherwise they would have had evidence. That is the reason they were blown up, because I don't think they realized they were going to be killed," he said.

In Pakistan, an intelligence official said Tanweer had been in the Lahore area from December to February and had stayed at a madrassah, an Islamic religious school of the type widely seen by security agencies as breeding grounds for militancy.