The bookshop on Bude Street never opened for business last week. The sign on the whitewashed storefront says IQRA THE LEARNING CENTRE, but Muslim neighbors in the trash-strewn south Leeds district of Beeston say the place is almost always closed. Some don't think it's a proper bookshop at all. They say that the young men who frequented the place were running a radical Islamist indoctrination center. "I don't know what they were trying to do," said a local schoolteacher who was afraid to be quoted by name, "but they took a lot of young, free people who were clubbing and dancing, and they changed them."

On Friday police raided the shop. They didn't say whether they found anything suspicious, and the owner could not be reached for comment. But no one doubted what the searchers wanted: evidence in the four coordinated rush-hour bombings that killed at least 55 bus and subway riders in London on the morning of July 7. The bookstore's neighborhood was once home to three of the four suicide bombers. Mohammed Sidique Khan lived a block away on Stratford Street when he was a boy, before his family moved to another neighborhood nearby. Shahzad Tanweer used to live just around the corner. Hasib Hussain lived minutes away. They all knew each other.

Beeston's residents seem as baffled as anyone else by what could possibly have transformed the local boys into terrorists. Investigators on four continents are now urgently trying to answer the question. They're increasingly convinced that the London bombers could not have planned or carried out their bloody mission without guidance from at least one trained terrorist--a "skill-set operative," in the jargon of counterterror experts. As evidence continued to pile up last week, investigators found it increasingly plausible that Al Qaeda might have had a direct hand in the bombings. "What we expect to find at some stage is that there is a clear Al Qaeda link, a clear Al Qaeda approach," said Metropolitan Police Commissioner Ian Blair.

British and American officials close to the inquiry (they can't be named for fear of compromising the investigation) say that as far as they can tell, none of the four ever attracted serious attention from British police or intelligence agents who track Islamic extremists. Yet two of the suspected bombers appear to have had at least tangential connections to terror suspects from an earlier case. The most mysterious member of the bombers' group has been identified by British investigators as Germaine Lindsay, 19, a Jamaican-born British citizen and Muslim convert who has lived both in Leeds and in the town of Aylesbury, roughly 170 miles away. Some investigators now think he may have been the team leader. How he might have joined up with the others is still unclear.

Some American officials, who render the Jamaican bomber's name as Jermaine Maurice Lindsay, say he was on a list of some 2,000 names collected last year in connection with Operation Crevice, a British and Pakistani joint police action aimed at foiling an alleged terror plot. It appears that Lindsay had contact with someone under scrutiny in the case, which concerned a scheme to blow up London's Heathrow airport or some other target of similar magnitude. But the Jamaican, who American investigators say visited his mother in Cleveland more than once during the 1990s, was never listed as a major terror suspect, and some British officials deny they ever had him in their sights. Investigators say Mohammed Sidique Khan's name also surfaced in the Operation Crevice inquiry, which ended with the arrest of eight British-born ethnic Pakistanis. (Those eight appear to be in custody, awaiting trial, although Britain's restrictive press laws have circumscribed media coverage of the story.)

Lindsay's alleged partners were a mixed bunch. Khan, 30, worked with special-needs kids at Hillside Primary School and was the married father of an 8-month-old daughter. "He was a nice bloke," said Fred Dibner, 35, clutching a tabloid newspaper with Khan's photo on the front page and gently leading his tearful 9-year-old son out of the school gates. "He helped Joe catch up after he missed six months of school." People liked 22-year-old Tanweer as well, says Abbas Hanif, a regular customer at the family-owned fish-and-chips shop where Tanweer worked as a counterman. "He was a nice guy, peaceful," says Hanif. "He liked sports." Hussain, 18, was a successful business student at a local vocational school. Neighbors said he once had a wild streak but became more disciplined after a trip to Pakistan a couple of years ago.

Tanweer also traveled to Pakistan in 2003 and again at the start of this year. Pakistani authorities believe he made contact with militants, including the outlawed Kashmiri independence groups Lashkar-i-Taiba and Jaish-i-Mohammed. Investigators say he also spent time at a madrassa near the city of Lahore, although they deny knowing whether the religious school has any jihadist ties. Six Pakistanis were detained last week in connection with the investigation. (The officials add that they have no record of Hussain's visit.) The Pakistanis claim that a tip from them fueled Operation Crevice, based on information found on the computer of Al Qaeda's alleged Webmaster in Lahore, Muhammed Neem Noor Khan, after his arrest a year ago. They also claim that this past May they gave London another warning about possible attacks after the capture and interrogation of senior Qaeda leader Abu Faraj al-Libbi near Peshawar. But a U.S. government official says that al-Libbi did not give interrogators specific information about this month's plot.

The hunt for one or more masterminds in the subway attacks has led to North Africa as well. British authorities issued a worldwide alert for Magdy Mahmoud Mustafa el-Nashar, 33, an Egyptian chemist at Leeds University who had not been seen on campus since early July. Investigators emphasized that he was wanted for questioning, not as a suspect. His rented home in the city's Hyde Park area is two blocks from a house that was described by police as a "bomb factory." A bathtub at the latter residence reportedly contained evidence of the dangerously unstable explosive TATP, the same stuff the Qaeda "shoe bomber" Richard Reid was wearing on his feet when fellow passengers prevented him from blowing up an airliner three days before Christmas 2001. A substantial quantity of the same explosive was discovered in a rental car that had been abandoned at the Luton railway station, near Aylesbury, on July 7.

Late last week the Egyptian government confirmed that Nashar had been arrested. He was just leaving afternoon prayers at the Tawhid mosque near his parents' home in Maadi, a Cairo suburb, when three police cars pulled to a stop. "Two men got out of one of the cars and went right for Magdy," says Ahmad Gamal, a 13-year-old family friend who watched it happen. "They took him into the car, and the others told everyone to keep walking." The chemist furiously protested his innocence, and Egypt's Interior minister categorically denied press speculation that Nashar had ties to Al Qaeda. In the United States, FBI agents visited North Carolina State University and served a subpoena on Peter Kilpatrick, head of the school's chemical- and biomolecular-engineering department, who handed over all available records pertaining to Nashar's brief stint as a chemical-engineering student there in early 2000.

Some U.S. and British officials suspect that the plot may have involved several facilitators, possibly a recruiter based inside the country and an operations expert from elsewhere who paid occasional visits to the bombers. The facts that have been uncovered so far suggest that the four were carefully screened and indoctrinated. Some townspeople, including Khan's family, said the young men had been "brainwashed"--but by whom? Leeds residents say the city's mosques are relatively moderate and decidedly nonviolent. Some neighbors point to the bookshop on Bude Street, while others blame militants who have gravitated to the nearby Hamara youth center, an interfaith meeting place created in 1995 by a consortium of two churches, a mosque and four other community groups.

Last Thursday the inhabitants of Beeston joined the rest of the country in two minutes of silence to honor the bombers' victims. The brief commemoration had barely ended when sirens pierced the stillness. Police motorcycles and bomb-disposal specialists roared down Lodge Lane to a building formerly used by the youth center. Police evacuated the area for several blocks around before setting off a controlled explosion and sending in a radio-guided robot. The center's director, Hanif Malik, denied that the bombers had any official part in administering its services. He confirmed only that the three men sometimes used the center's gym facilities.

Deep beneath the streets of London, workers continued to sift through the wreckage. British authorities warned that unraveling the plot was likely to take months, if not longer, and they begged the public for patience. Some individuals clearly weren't listening. Mosques across Britain received bomb threats last week, and racist Web sites called for a campaign of violence against Muslims. Jihadists could only rub their hands at the thought.