LEEDS, England – Police pursued what they suspect is a mastermind behind London’s terror attacks, raiding a home yesterday and widening their search to a new area. The top law enforcement official suggested the bombers were “foot soldiers” intent on causing mayhem.

Last night’s raid in Aylesbury, about 40 miles northwest of London and near the city of Oxford, resulted in no arrests, but police were searching the house, Scotland Yard said.

As a show of defiance, London’s trademark black taxis and red doubledecker buses were asked to pull to the side of the road and workers were urged to take to the streets at midday today for a moment of silence marking the week that has passed since the July 7 terrorist bombings that killed at least 52 people.

Details emerged yesterday about the lives of the suspected bombers, one of whom was only 19 years old. But authorities suspected the four attackers – including at least three Britons of Pakistani descent – didn’t work alone, and that their collaborators or leader were likely still at large.

“These foot soldiers who have done this are only one element of an organization that is bringing about this kind of mayhem in our society,” the country’s top law enforcement official, Home Secretary Charles Clarke, told the British Broadcasting Corporation. “We are looking very, very closely at the relationship between the people who may have committed the offenses and the wider network around them.”

Mr. Clarke said nations needed to defend their core group of values “against those who would destroy it.”

“That means standing out against, in a very strong way, anybody who preaches the kind of fundamentalism, as I say, which can lead four young men to blow themselves and others up on the tube on a Thursday morning,” he said.

His comments went beyond the cautious statements of police, who said Tuesday that they were investigating the possibility that all four suspects died in explosions on a bus and three subway trains.

The Home Office said there was no evidence yet to support a conclusion that the bombers intended to die.

An American government official confirmed yesterday that Shahzad Tanweer, Hasib Hussain, and Mohammed Sidique Khan are thought to have been three of the bombers. The names are being checked to see if they appear on any American databases. The official spoke on condition of anonymity because British investigators have not publicly released the identities of the suspected attackers.

Britain’s Press Association, citing police officials, said yesterday that police had identified the fourth suspected bomber but no details were reported.

Surveillance cameras captured the four as they arrived in the capital 20 minutes before the beginning of the rush-hour explosions. Police refused to comment on a BBC report that authorities were seeking a fifth suspect.

In London, police searched through evidence from close-circuit TV footage and the scene where the blasts ripped apart trains and a bus. In and around the city of Leeds, home to a large Muslim community, police searched for evidence in neighborhoods where three of the four suspects lived.

More than 100 alleged revenge attacks – including the killing of a Pakistani immigrant – have been reported in Britain since the bombings.

With fears of further backlash growing, Prime Minister Blair urged Britons not to judge all Muslims by the acts of those inspired by a “perverted and poisonous misinterpretation” of Islam. Addressing the House of Commons, Mr. Blair said the government also would look at how to strengthen the process for excluding from Britain those who incite hatred.

In a series of police raids on six homes in the Leeds area on Tuesday, police hunted for explosives and computer files that could lead to a mastermind behind the London attacks. They arrested a 29-year-old man identified by Press Association as a relative of one of the suspected bombers.

One of the alleged bombers, Shahzad Tanweer, was a 22-year-old; Hasib Hussain was 19, and Mohammed Sidique Khan was the 30-year-old father of an 8-month-old baby.

Tanweer’s uncle, Bashir Ahmed, said his nephew had gone to Pakistan for two months earlier this year to study religion, and that the family believed he was attending “some religious function” on the day of the bombings.

“Our lives have been shattered,” he said. “It’s impossible to describe it. We have had a very pleasant time here. I don’t think we can continue here.”