Police hunted Wednesday for a fifth conspirator in the London transit system bombings last week and raided a home in Aylesbury, a suburb north of the capital, as the investigation widened into how the attacks were planned and who might have inspired them.
Detectives interrogated for a second day a man from Leeds described as a relative of one of the bombers and searched for explosives in houses where he had lived, officials said. Police continued to comb through a house where they found a small cache of explosives Tuesday in another part of Leeds, an industrial city about 200 miles north of London that investigators believe was home to at least three of the four bombers.
In the United States, U.S. law enforcement authorities were questioning Richard C. Reid, a British citizen serving a life sentence for attempting to blow up an American Airlines jet in 2001, for any leads he might have on the bombers, a U.S. intelligence source said, speaking on condition of anonymity.
British newspapers reported in their Thursday editions that investigators believed the fifth suspect was a possible organizer of the cell, but feared he might have fled the country shortly before the bombings. Police did not release a name, but U.S. intelligence sources said law enforcement authorities knew the person's identity.
As law enforcement officials raced to reconstruct the genesis of the plot, the environment in which the alleged suicide bombers lived was easy to see Wednesday in the working-class immigrant neighborhoods of Leeds. Police tape cordoned off the city blocks where, only a few days ago, the men had hung out on the sidewalks with unsuspecting neighbors. Technicians in white biohazard suits poked through the men's homes, looking for clues.
The most visible police operations were in the Leeds suburb of Beeston, home to Shehzad Tanweer, 22, a college dropout and cricket devotee whose family has acknowledged he was one of the bombers. Friends and relatives said he had visited Pakistan for a few months this year to study Islam but was a familiar face in the area, working part time at his father's fish-and-chips shop nearby. "He was a nice guy, always friendly," said a 12-year-old boy who lives down the street.
Tanweer's uncle, Bashir Ahmad, told reporters outside the family's South Leeds Fishery restaurant that they had thought Tanweer was attending a "religious function" on the day of the bombings and still could not comprehend what might have driven him to kill innocent strangers. "It was total shock -- I mean it was unbelievable," Ahmad said. He said Tanweer's parents were "broken" and predicted they would move from the area soon.
A short stroll from Tanweer's home is a storefront mosque where residents said he frequently met with two of the other alleged bombers, including 18-year-old Hasib Hussain. Neighbors said Hussain had become visibly more religious about two years ago and traveled to Pakistan. They said he struggled in high school and was unemployed.
British investigators believe Hussain might have tried to board a Northern Line subway train at King's Cross station Thursday but given up because of severe delays on the line, said an official who insisted on anonymity. He instead boarded a double-decker bus, where he detonated his explosives nearly an hour after the three other blasts.
Had the original plan worked, the four bombers would have created a flaming cross of attacks radiating north, south, east and west from King's Cross. This fits the original assertion of responsibility by a group calling itself the Secret Organization of al Qaeda in Europe, which said in a Web site posting that "Britain is now burning with fear, terror and panic in its northern, southern, eastern and western quarters."
The message has not been authenticated. Another group purportedly affiliated with al Qaeda, the Abu Hafs al-Masri Brigade, has also asserted responsibility.
British news media have given conflicting names for the third and fourth men, one of whom police have said lived in the neighboring town of Dewsbury. The fourth bomber is said to have been from the Leeds region.
That Leeds-area men were plotting the worst terrorist attack in British history apparently went unnoticed in the city's immigrant neighborhoods, a religious, ethnic and racial melting pot of Anglos, Bangladeshis, Indians and Pakistanis, with a lesser number of black immigrants from Africa and the Caribbean. Leeds has about 30,000 Muslims -- roughly 5 percent of the population.
Religious and civic leaders in Leeds held an emergency meeting Wednesday in a community center two blocks from Tanweer's home to discuss how to respond to the news. Several Muslim leaders said the city had no history of Islamic extremism or proselytizing by radical groups.
"As a lifelong resident of Leeds, I can honestly say that we've never come across any radical elements," said Hanif Malik, director of the Hamara Center, which is housed in a century-old Anglican church that was converted to a cultural center and serves a predominantly Muslim population. "Since this broke, the biggest mystery has been exactly that: How can a community like ours harbor perpetrators like that?"
Religious and civic leaders issued a public statement condemning the bombings. "I predict there will not be a backlash in this community against Muslims," said the Rev. Neil Bishop, an Anglican minister in Tanweer's neighborhood. "But I hope no one will come here from somewhere else to cause trouble."
Residents of the Hyde Park neighborhood, home to one of the city's largest mosques and where about 300 people remained displaced from homes by the searches, said they were planning a "prayer walk" Wednesday night to demonstrate resolve for tolerance. "Hyde Park is one of the most multicultural societies in all of England," said Pat Regan, 50, a walk organizer and Leeds native. "We've always had a tight community, and we've got to keep it that way."
Mohammed Siddiq, owner of an Indian restaurant in Hyde Park, said that the neighborhood had always been peaceful but that he was getting worried. Since the attacks of Sept. 11, 2001, he said, there has been a lot of pressure on Muslims, and anger in the community over the Israeli-Palestinian conflict and the war in Iraq. "I don't want any trouble, but maybe it's made some people think differently," he said. "Maybe we have a problem now with some people."
Linzer reported from Washington. Correspondent Glenn Frankel in London and researcher Julie Tate in Washington contributed to this report.