"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.
