Four young British citizens of Pakistani origin appear to have carried out last week's bombings of the London transit system, blowing themselves up along with their victims in what would be the first suicide attacks in Western Europe, British police said Tuesday.
In a day of fast-moving developments, heavily armed police backed by army units raided six houses in and around the northern city of Leeds seeking evidence, arrested a relative of one of the suspects and carried out a controlled detonation of explosives at one of the sites.
The preliminary findings against native-born Muslims brought expressions of anguish from a prominent Islamic community leader, as well as warnings from security officials that other people involved in the plot were probably still at large and dangerous, including the presumed bombmaker behind the operation. Britain remained on its highest-ever security alert.
Investigators say they believe the men drove to the Luton train station 30 miles north of the capital early Thursday morning and boarded a train to King's Cross station in north London. They were recorded there on closed-circuit security cameras just before 8:30 a.m.
Three of them boarded subway trains heading in different directions -- one toward Liverpool Street and Aldgate, one toward Edgware Road and one toward Russell Square. Bombs blew up on those three trains within 50 seconds of one another about 8:50 a.m.
The fourth man wound up on a No. 30 double-decker bus that was diverted to nearby Euston station after the explosions shut down King's Cross. His bomb detonated nearly an hour after the others at nearby Tavistock Square, killing at least 13 people.
In total, at least 52 people were killed and about 700 wounded.
The Sept. 11, 2001, hijackers used suicide attacks against targets in the United States; the tactic has become common in Iraq and other parts of the Middle East. But it has not been used before in Western Europe. The attacks on commuter trains in Madrid in March 2004 were carried out with planted bombs. Seven suspects later blew themselves up after being cornered by police; a police officer also died in that blast.
Counterterrorism officials in Europe have long feared that radical groups on the continent would eventually embrace suicide attacks as a strategy, copying a tactic that has been used widely against Israel for years.
"It's a pretty dramatic thing," said Evan F. Kohlmann, a New York-based terrorism consultant and researcher. "The British had felt that this kind of attack was inevitable, that it would happen no matter what. They had stopped a number of these guys before, but some finally got through."
Officials say they believe that the three men who attacked the London trains intended to kill themselves. Personal documents bearing the names of two of the men were found close to the seats where two of the train explosions took place.
Officials believe that those two men died on the trains. They suspect that the body of the third train bomber may be trapped in the wreckage of a subway car in the underground tunnel between the King's Cross and Russell Square stations.
"They have to literally piece people back together, and that takes time," said a police official who insisted upon anonymity.
Officials said they were not certain about the intentions of the bus bomber. Witnesses have reported seeing a young man fidgeting nervously with something inside a bag just seconds before a bomb exploded in the area where he had been.
Peter Clarke, head of the counterterrorism branch of Scotland Yard, told reporters that investigators, who have conducted a painstaking fingertip search of the bus site, found property they traced to the suspect and were certain he died in the explosion.
British police released no names for any of the four. But neighbors in Leeds named one of the alleged bombers as Shahzad Tanweer, 22. According to British newspapers, the others were Hasib Hussain, 18, Rashid Facha, in his twenties, and Jacksey Fiaz, about 35.
Tuesday brought a dramatic breakthrough in what Clarke called "a complex and intensive terrorist investigation moving at great speed."
After five days of investigation, detectives early Tuesday had put together several key pieces of evidence: the missing-persons report from the family of one of the suspected bombers; the recovered identity documents; and security camera footage at King's Cross of four men with backpacks on the morning of the bombings.
Investigators obtained search warrants for the houses of three suspects and three other residences in and around Leeds, one of Britain's largest cities, about 170 miles north of London. Dozens of local police and counterterrorism detectives, backed by an army unit, entered three blue-collar neighborhoods, sealing off the areas and evacuating about 600 residents.
Investigations continued in Luton as well. In a parking lot at the train station there, dozens of officers surrounded a car that investigators suspect was used by the bombers. Officers set off remote detonations of explosives found inside the vehicle.
Many questions remained following Tuesday's disclosures. Police had originally reported that explosives in backpacks had been placed on the floor near train doors, suggesting that the people who put them there had intended to escape and not commit suicide.
Police also gave no explanation for where the suspects had obtained the high explosives -- 10 pounds in each of the backpacks -- that they reportedly used. And they did not explain whether the bombers had used timing devices or set off the explosives by hand.
While police would not elaborate publicly on the statement they released, investigators told reporters after the briefing that they believed that the bombers had support from other conspirators and that it was likely some of that support came from abroad.
Still, the profile of the suspects suggested by investigators fit long-standing warnings by security experts that the greatest potential threat to Britain could come from second-generation Muslims, born here but alienated from British society and perhaps from their own families, and inflamed by Britain's participation in the Iraq war.
A government memo published in the Sunday Times last weekend warned that a loose group of "extremist recruiters" sympathetic to the al Qaeda network was targeting susceptible young Muslims, especially those with technical and professional qualifications in engineering and computers. Most did not have police records, it said.
On Tuesday, Iqbal Sacranie, leader of the Muslim Council of Britain, told the BBC he had received the latest news from the police with "anguish, shock and horror."
"It appears our youth have been involved in last week's horrific bombings against innocent people," he said, reiterating the Muslim community's commitment to tracking down the killers. "Nothing in Islam can ever justify the evil actions of the bombers," he said.
Andy Hayman, assistant police commissioner, emphasized to reporters that the Muslim community should not be blamed for the bomb attacks. "No one should be in any doubt the work last Thursday is that of extremists and criminals," he said. "That being the case, no one should smear or stigmatize any community with these acts."
Officials formally identified 10 more victims of the bombings, bringing to 11 the total of confirmed dead. They said the work had been hampered by hot and difficult conditions at the Russell Square bomb site and by the need to preserve evidence in what Commissioner Ian Blair of Scotland Yard called "the biggest crime scene in British history."
The State Department on Tuesday identified an American who was earlier declared missing and presumed dead in the bombings as Michael Matsushita, of Bronx, N.Y.
Also Tuesday, local U.S. Air Force commanders came under sharp criticism for ordering American military personnel at two air bases in southeast England to stay away from London because of the security threat. The Daily Mail said the travel ban had handed "a symbolic victory to terrorists," and British Defense Secretary John Reid promised to look into the measure.
By noon, the ban had been lifted under direct orders from the commander of U.S. forces in Europe, Marine Gen. James L. Jones. "It was a little overblown, but at the same time there is a sensitivity to the public communications issue," said David Johnson, charge d'affaires at the U.S. Embassy.
Correspondent Craig Whitlock in Leeds contributed to this report.