Some Details

LONDON, July 7 - Investigators searching for clues in the attacks here said Thursday that the three bombs used in the subway apparently were detonated by timers, not suicide bombers, and that a fourth device may have been intended for a target other than the city bus that it destroyed. (snip)
A group calling itself the Secret Organization of Al Qaeda in Europe announced on a Web site that it was responsible for the bombings. The announcement also threatened Italy and Denmark, which have provided troops to the American-led coalition fighting in Iraq.

The authenticity of the message could not be confirmed, and several experts said they strongly doubted that it was authentic. (snip)
Counterterrorism officials in London said they were still trying to determine the type of explosives that were used. One official speculated that the No. 30 bus whose roof was blown off at 9:47 a.m. in Bloomsbury was demolished accidentally by a suicide bomber. But another theory gaining momentum was that the bomb exploded prematurely as a bomber was carrying it to an intended target, several American and British counterterrorism officials said.

The officials said that the three subway bombs appeared to have been detonated by timers, not cellphones or other remote triggers. The bombs on the trains were believed to be package bombs and are believed to have been left by the attackers who fled before they went off.

Officials refused to confirm or deny reports that two unexploded package bombs were recovered from trains. A senior American intelligence official said the British had conducted "at least one controlled explosion" of a suspicious package found after the attacks, but he said he could not confirm that the package was another bomb.

The bombings in Madrid and London were separated by 16 months, and the ones in Madrid were set off by cell phones. But the attacks bear eerie similarities and grim lessons for counterterrorism officials. (snip)
"There have been a lot of attempts" in London, said Baltasar Garzón, a Spanish judge who has investigated Al Qaeda for years. "And in this case, they finally hit some of the easiest targets with these trains. Their only real obstacle to this kind of action is getting the explosives. Once they have them, it's very easy to attack targets like Sunday trains."

Mr. Mates, of the Intelligence and Security Committee, said it had been only a matter of time before a coordinated attack struck London. He said the authorities had thwarted at least three coordinated attacks in London since 9/11. "We've caught and prevented those who were trying to get through and stopped them," he said.

Britain has considerable experience investigating bombs and identifying those responsible, based on years of attacks in London and in Northern Ireland by the Irish Republican Army.