The bombs that ripped through three London subway trains during Thursday's morning rush hour were high explosives triggered almost simultaneously, most likely by synchronized timing devices rather than by suicide bombers, according to a revised account released on Saturday by police.
After examining computer data, electrical equipment and eyewitness statements, investigators have concluded that the three bombs were set off within 50 seconds of one another, officials said during a news conference. "It was bang, bang, bang -- very close together," said Tim O'Toole, managing director of the London Underground.
The disclosure came on a tense day as officials closed off parts of central Birmingham, Britain's second largest city, due to an unspecified security threat, and authorities warned that the bombers may still be at large. About 20,000 people were evacuated, and police carried out four controlled explosions of suspicious items, but officials later said they believed the alert was not connected to the London bomb attacks.
Officials had originally reported a 26-minute gap between the first and third bombs. They attributed the discrepancy to the fact that no one phoned in the explosion at the Edgware Road station as a bomb until 9:17 a.m. -- some 25 minutes after it actually occurred.
The three blasts, and a fourth explosion that tore through a double-decker bus nearly an hour later, killed at least 49 people and injured 700. Another 25 people are missing and feared dead, most of them entombed in the wreckage of a Piccadilly Line train in a narrow tunnel some 70 feet below ground.
The new details match the leading theory that is emerging among investigators, analysts said: that the bombings were a technically competent and well-coordinated attack planned and overseen by at least one experienced and well-trained operative using commercially manufactured explosives, and carried out by local people.
Such a pattern would fit previous bomb attacks in Casablanca in May 2003 and Madrid in March 2004. In each case, an operative from outside the country trained in Islamic extremist camps in Afghanistan or Pakistan used local people with no known links to terrorism to carry bombs to their targets. Irish Republican Army guerrillas used similar local helpers -- known here as "lily whites" because they had no police record -- in their decades-long bombing campaign against British rule in Northern Ireland.
"Their major hypothesis is that this is basically a home-grown group with external help or inspiration," said Michael Clarke, director of the International Policy Institute at King's College London, who has close ties to security officials. "A plausible pattern is that professionals from abroad come in and set the thing up and the lily whites go and do it."
Prime Minister Tony Blair defended the security services from allegations that they had been caught by surprise by Thursday's attacks. The government was doing everything it could to protect the country, he told the BBC, but "if people are actually prepared to go onto a tube or a bus and blow up wholly innocent people, people just at random, to do the maximum death and destruction without any thought for their human rights or human life, you can have all the surveillance in the world and you couldn't stop that happening."
Two Islamic extremist groups have asserted responsibility for the attacks, which brought central London to a standstill.
Meanwhile, rescue workers and forensic experts continued their painstaking effort to extricate the dead and gather evidence from the Piccadilly train, located between the King's Cross and Russell Square stations, scene of Thursday's largest death toll.
Contrary to earlier reports, officials said the tunnel was undamaged. But they said heat, lack of ventilation, tight space, vermin and the trauma of working among mangled bodies made the task slow and horrific.
"It is extremely hot, very dusty and quite dangerous down there and it is a great challenge for them to continue their work to recover the remaining bodies," said Andy Trotter, deputy chief constable of the British Transport Police. "It will be some time before this job is completed."
Officials refused to say how many bodies had been found or how many more might be enclosed in the rubble. They said that none of the victims had been officially identified.
"This was a high explosive detonated in a very confined space," said Brian Paddick, a senior constable in Scotland Yard, headquarters of London's police force. "You can quite clearly understand why it is not absolutely clear how many people may be in that carriage." The work has gone on out of sight of the public and media, which has had no glimpse of the shattered train. But British newspapers carried accounts of rescue workers who first came upon the carnage.
"I got into the train and it was quite obvious that this was something horrendous," Sgt. Steve Betts of the British Transport Police told the Guardian newspaper. "There were body parts everywhere, there was not one bit as far as I could see that was not covered with organs or blood or bits of body. I was squashed in by chairs and dead bodies as we searched for anyone alive."
At King's Cross, flowers and sympathy cards were piled against a wall to honor the dead. Posters of the missing put up by relatives or friends remained scattered about.
Several hundred people gathered at a churchyard late Saturday for a memorial, a block from the station, most of which is still shut down, with high screens blocking investigators' work from public view. Mourners observed a minute of silence interrupted only by the whine of passing buses, then speakers launched into calls for the withdrawal of Britain's troops from Iraq.
Leaders of both of Britain's largest political parties supported the Iraq war, and politicians generally have rallied around the government following the bombings. One of the few to speak out against the government in recent days has been George Galloway, a vociferous antiwar critic and member of Parliament.
"The truth is we have been dragged into a place of great danger, and many have paid with their lives and limbs," he told the crowd. Galloway cited the victims still lying in a tunnel near the site of the vigil, and the bus passengers who "tumbled to their death" in another bombing on an adjoining block.
Analysts pieced together information from the Underground's software and electrical systems to help come up with the revised portrait of Thursday's attacks. O'Toole said the first bomb struck the No. 204 Circle Line train traveling from Liverpool Street to Aldgate station at 8:50 a.m., the height of the city's morning rush hour. Within seconds, the No. 216 train pulling into Edgware Road was also hit, damaging another train on an adjoining track. Then came the worst attack, with the explosion of the Piccadilly Line train heading to Russell Square from King's Cross.
London Underground officials did not declare a "Code Amber" alert and shut down the system and evacuate all passengers until 9:15 -- 25 minutes after the blasts. "To stop the system is a very, very brave decision," said O'Toole. "It requires exquisite timing by your staff to get everyone out safely."
Paddick, of Scotland Yard, said the synchronized detonations meant that either the bombers set off the explosions using watches or, more likely, the bombs were triggered by coordinated timing devices. The devices were in "millions of pieces" at the bomb sites, he said, but eventually would be located and pieced together.
Because the bombs, weighing around 10 pounds each, were located in bags placed on the floor or seats near the doors of the subway cars, officials said they had no evidence to suggest the attacks were the work of suicide bombers. They offered no explanation for the fourth bomb that struck the bus nearly an hour later.
In Italy, police said they had arrested 142 people in a two-day anti-terrorism security sweep around Milan prompted by the London bombings, the Associated Press reported. Many Italians fear their country may be next on the hit list of the al Qaeda network because of the government's support for the U.S.-led invasions of Afghanistan and Iraq, and a senior police commander said the roundup was designed to make people "feel calmer after the London attacks."
Meanwhile, a British citizen who has been accused by Moroccan authorities of leading an extremist network called the Moroccan Islamic Combatant Group, surfaced by giving an interview to al-Jazeera television. Mohammed Guerbouzi, who many European newspapers had reported was being sought by investigators, said he was keeping a low profile in Britain because he was afraid of being harassed. He said Scotland Yard investigators knew of his whereabouts.
Correspondents Ellen Knickmeyer and Craig Whitlock contributed to this report.