A phone call from an anguished mother searching for her teenage son after London's terror attacks gave investigators a key break in the case.

The eventual answer to her query was even worse than she had dreaded — her son is suspected of being one of four bombers who hit three subway trains and a bus on July 7, killing at least 52 people.

The woman's description of her missing son's clothes matched those on a body whose horrific injuries — including decapitation — suggested he could have been the suicide bomber who blew up a red double-decker bus, killing 13 people, The Times newspaper reported Wednesday.

Detectives examining hundreds of hours of closed-circuit television tape reportedly were told to look for the man, identified in press reports as Hasib Hussain, 19. They found the crucial images of him and three others Monday night, and pounced early the next morning in a series of raids near Leeds, 185 miles north of London. Among homes targeted were three where the suspects had lived. At least three of the suspects were reportedly British citizens of Pakistani descent.

Suspects' personal documents, found in the wreckage of three of the four bomb blasts, filled in another piece of the puzzle, said Peter Clarke (search), head of the Metropolitan Police anti-terrorist branch. News reports identified the documents as credit cards and IDs.

Police investigating the attacks felt they were racing against the clock, fearing before it became apparent the bombers had died that they could strike again. That pressure hasn't dissipated — authorities worry the attackers didn't act alone, and that their collaborators or leader are likely still at large.

"We are looking very, very closely at the relationship between the people who may have committed the offenses and the wider network around them," said Home Secretary Charles Clarke, Britain's top law enforcement official.

Police said from the start that closed-circuit footage and forensic clues uncovered by detectives scouring the wreckage of the ruined trains and bus would be critical to their inquiry.

Detectives on Saturday revised their initial timeline of events, ascertaining from a review of the subways' computer and electrical systems that the bombings on three Underground trains occurred within a minute of one another, at 8:50 a.m., not over a 26-minute span as initially thought.

Along with the information from Hussain's mother, the new timeline helped them home in on the most relevant sections of the hundreds of hours of footage they'd collected. Cameras monitor much of central London; there are 1,800 of them in train stations and 6,000 in the Underground network.

At about 8 p.m. Monday, investigators found the images they were looking for.

Four young men carrying backpacks chatted casually in King's Cross station at about 8:30 a.m. — 20 minutes before the subway explosions — and then separated. At rush hour in the busy station, a major hub for north London, they would have attracted little notice.

Authorities initially thought Hussain might have been a victim of the attacks and sent family liaison officers to meet his parents, who had believed he was planning a day in London with his friends. Police likely obtained photos and other identifying details to help track the teen — pictures that later matched the closed-circuit TV images.

Police reportedly found Hussain's driver's license and bank card in the wreckage of the No. 30 bus that blew up at 9:47 a.m. in Tavistock Square (search), about a mile from King's Cross.

Some believe he may have intended to set off his bomb on the Northern Line, which also passes through the station, but hit a bus instead because a technical problem had disrupted trains on the line earlier that morning.

Documents belonging to two of the other suspects were found at two of the subway scenes, police said. Those suspects were identified in the media as Shahzad Tanweer, 22, described as a cricket fan; and Mohammed Sidique Khan, 30, a new father.

The third subway bombing site, a Piccadilly Line train deep beneath London's streets, is more severely damaged and harder to reach, and searchers have not yet linked anything found there to the fourth bomber. Britain's Press Association news agency, citing police sources, said police have identified the fourth suspect but did not report a name.

Prime Minister Tony Blair congratulated police and intelligence teams Wednesday for work he called "magnificent."

But they now face another daunting task — finding any collaborators who worked with or directed the young men.

Police pursued several leads Wednesday, seeking what they suspect is a mastermind behind the attacks. They also raided a home in Aylesbury, some 40 miles northwest of London and near the city of Oxford, but made no arrests, Scotland Yard said.

"I just don't believe this would have been four young men acting on their own," said Paul Wilkinson of the Center for the Study of Terrorism and Political Violence at St. Andrews University in Scotland. "There would have been another person who primed and guided them and lured them into extremism."

That mastermind could be part of a known terror network such as al-Qaida, possibly coming to Britain months ago to seek recruits and provide explosives for an attack.

"It's very clear that there are established links between (extremists) outside and inside" Britain, said Magnus Ranstorp, director of the St. Andrews center.

The theory that the bombers were aided by a sophisticated outsider may be bolstered by police's reported discovery of a "bomb-making factory" at one of the homes they raided in Leeds.

Police will surely seek clues to such connections as they question a man they arrested there. Press Association identified him as a relative of one of the suspects.

Reports say the bombing suspects were "clean skins," or people previously unknown to intelligence services. Investigators believe they traveled from Leeds to London early on July 7. News reports say they drove a rented Nissan Micra to Luton — a town 30 miles north of London where police have found explosives in a car — and then took a commuter train to King's Cross.

An employee of the rental car company showed up at one of the suspects' homes to reclaim the unreturned vehicle as police were searching the premises Tuesday.