LONDON — As distraught families protested the slow recovery and identification of bodies from Thursday's attacks, police disclosed yesterday that the bombs used in the three doomed subway trains were detonated within seconds of each other — a far more chilling level of synchronization than originally believed.

And in a sign that this remains a country on edge, the second-largest city in Britain was thrown into panic by a security alert last night. Police acting on an unspecified "credible threat," possibly a suspicious package on a bus, evacuated 20,000 people from the center of Birmingham, a teeming industrial city two hours north of London.

In London, authorities said bodies of severely mauled bomb victims might not be identified for days. The death toll stood at 49 but could rise significantly. Many bodies remain trapped in a rat-infested tunnel 100 feet below ground.

Scotland Yard said yesterday no suspects had been arrested, raising fears that the culprits were free to carry out additional attacks. At this point after last year's Madrid, Spain, train bombings, several key suspects were in custody.

Scotland Yard also announced that examination of electronic evidence confirmed that three of the four rush-hour bombs exploded within 50 seconds of one another, not the half-hour previously reported.

The revelation bolstered the theory that timers were used to detonate the devices, an important clue in establishing the method and eventually the identities of the bombers. It suggests the suspected Islamic extremists who carried out the attacks set the bombs to go off at the same time and then would have been able to deposit the devices, thought to be contained in backpacks, in the trains before escaping.

"It was bang, bang, bang — very close together," Tim O'Toole, managing director of the London Underground, said of the near-simultaneous blasts.

The use of simultaneous bombs also means there would be no time to alert travelers and save lives.

"High explosives"

Police previously said that each bomb weighed less than 10 pounds and was deposited on the floor near the door of each of three subway carriages on the Piccadilly and Circle lines. Yesterday, they added that the bombs were composed of "high explosives," meaning something such as TNT or plastics, not a homemade brew of chemicals.

Deputy Assistant Police Commissioner Brian Paddick declined to identify the material further and cautioned against reaching conclusions at this early stage in the investigation.

The fourth bomb, which detonated on a double-decker bus 57 minutes after the first blasts, remains the mystery. Possibly it was meant as a second-wave attack timed to hit people fleeing the first wave, or the bomber somehow failed to reach his or her destination and the explosion was a mistake.

European assistance

British investigators told their European counterparts, arriving in London yesterday to assist in the probe, that they are leaning toward the latter theory.

The evidence so far suggests a fairly simple type of bomb but a detailed degree of planning and execution by people who had knowledge of London's transit system. Authorities are investigating whether the deadliest terror attack on British soil was the work of any of the several Islamic cells that have formed in London in recent years but until now had not attacked domestically.

Investigators said they had not narrowed their manhunt to a specific nationality or ethnic network, but several European officials said they were convinced that a Moroccan-dominated group was responsible. Moroccan extremist groups have been especially active in Europe, having taken the lead role in the Madrid bombings and in the assassination in November of Dutch filmmaker Theo van Gogh.

Focus of suspicion

Several top Moroccan planners of the Madrid bombings, which killed 191 people, remain at large and were tracked at one point to London. In that vein, suspicion has focused on Mohammed Gerbouzi, who has lived in London for years despite court charges in Morocco identifying him as a top figure in the Moroccan Islamic Combatant Group, a network implicated in the Madrid and Amsterdam cases as well as suicide bombings in Casablanca, Morocco, in 2003.

Gerbouzi "is probably not involved directly with the British attacks because he felt targeted, but there may be people in his circle who have participated," said Louis Caprioli, the former head of France's anti-terror DST intelligence service.

Another possible suspect connected to the Madrid case as well as to the Sept. 11 attacks in the United States is Mustafa Setmarian, a Syrian-born alleged al-Qaida operative who ran training camps in Afghanistan and then purportedly set up a terror network in Spain. His whereabouts is unknown.

As the search for the attackers continued, increasingly desperate families pressed for information on missing loved ones and complained bitterly over the government's failure to provide lists of the dead and injured.

A New Yorker who left the city four years ago to see the world was feared dead in the attacks, a relative in the Bronx and friends in New York and London said yesterday.

The missing man, Michael Matsushita, 37, who moved to London recently to be with his fiancée after working as a recruiter in Australia and as a tour guide in Southeast Asia, was believed to be aboard one of three subway trains torn apart by bombs that killed at least 49 people and wounded more than 700.

Matsushita's name was not on official lists of the dead or injured, and the State Department had no reports of any Americans killed in the attacks, although four were listed among the injured. But dozens of victims were still unaccounted for in the wreckage, and it appeared that Matsushita might be the only American killed in the carnage.

Deliberate process

Authorities are moving at a slow pace in identifying bodies and giving formal notification to families. A police official in charge of body recovery said it could be weeks before the names of the dead are released, adding that priority had been given to the collection of forensic evidence to aid in the criminal investigation.

The official, Detective Superintendent Jim Dickie, said only the 13 bodies recovered from the double-decker bus have reached the medical examiner's morgue where autopsies will be performed. Many victims, perhaps 20 to 25, remain more than 100 feet underground in one shattered train carriage between the King's Cross and Russell Square stations.

Many of the nearly 70 wounded still in hospitals are amputees, Dickie added.

The slow pace is attributable, in part, to the grueling recovery operation at Russell Square, the deepest and most narrow section of Tube tunnel stricken in the attacks. Crews in white hazmat suits must descend slowly into the cramped tunnel, where the air is heavy with dust, heat and the fumes of decomposing corpses.

Resilience lauded

Despite the scant information from officials, a picture has emerged of the dead and injured — of native-born Britons and immigrants from across the globe, of numerous religious faiths, all workaday Londoners caught on their way to jobs, school or shops.

Prime Minister Tony Blair, in a radio interview yesterday, again praised the resilience of the British people in the face of an onslaught he said was expected and inevitable.

"As a government you have to do everything you can to protect your people," he said, "but if there are people prepared to go on the Tube or a bus and blow up wholly innocent people ... you can have all the surveillance in the world and not stop it from happening."

Los Angeles Times reporters Sebastian Rotella and Janet Stobart contributed to this report. Information on the missing U.S. man was from The New York Times.