The bombs that destroyed three London Underground (search) cars and a double-decker bus each weighed less than 10 pounds and could be carried in a backpack, police said Friday.
An explosives expert said they were likely crude homemade devices set off with a simple timer. Experts say Thursday's attacks had all the hallmarks of an Al Qaeda (search) strike, and authorities were gathering evidence on the ground and investigating a purported claim of responsibility.
Sir Ian Blair, commissioner of London's Metropolitan Police, said no arrests had been made but officials have "lots and lots" of leads.
Home Secretary Charles Clarke, the Cabinet minister responsible for law and order, said it was a "strong possibility" that Al Qaeda (search) or a sympathetic group had carried out the attack.
In Washington, current and former American counterterrorism officials said they were taking seriously an Internet claim by a little-known group calling itself The Secret Organization of Al Qaeda in Europe that it staged the attacks.
A U.S. law enforcement official said authorities had vague information from Abu Farraj al-Libbi (search), reputedly No. 3 in the Al Qaeda terror network, that Al Qaeda was seeking to mount an attack similar to the 2004 train bombings in Madrid.
Al-Libbi was arrested by Pakistani agents on May 2. The information contained no specifics about location or timing, the official said.
The bombs were probably made from simple, relatively easy-to-obtain plastic explosives, not the higher-grade military plastics like Semtex that would have killed far more people, said Andy Oppenheimer, a weapons expert who consults for Jane's Information Group.
"Any crook with ready cash could obtain this stuff if they knew where to look for it," said Alex Standish, the editor of Jane's Intelligence Digest.
Plastic explosives are readily available on the black market in the Czech Republic and other central and eastern European countries or through the Russian mafia, Standish said. Large amounts of plastic explosives untagged by the chemical markers that enable dogs to detect it are missing from Czech stocks, he added.
Police said the four bombs that hit the London transportation network on Thursday weighed less than 10 pounds each, small enough to be carried in a backpack. They were left on the floor of the Underground trains and either a seat or the floor of the No. 30 bus that was ripped apart in the Bloomsbury neighborhood, said Assistant Police Commissioner Andy Hayman.
Ten pounds is a relatively small bomb, although a blast's power depends more on the type of explosive than the amount. The 10 bombs that killed 191 people on commuter trains in Madrid, Spain last year averaged 22 pounds each; the bombs that killed 33 bystanders and 12 suicide attackers at five targets in Casablanca, Morocco, two years ago were 18 to 22 pounds each.
Hayman said investigators had so far obtained little detailed forensic information on the bombs. Their investigation has been hindered by the inaccessibility of one of the wrecked trains, 70 feet below street level, he said.
Bodies were still trapped in the mangled Picadilly line train between theKing's Cross and Russell Square stations, the site where at least 21 people were killed.
Rescuers got all the survivors out in the hours after the blast but decided not to go back to remove the dead or recover evidence until they can shore up the tunnel, which sustained structural damage and may be unsafe, said Blair, the police commissioner.
Oppenheimer said the bombers likely used a fairly basic timer that would have been set a half hour or less in advance. More sophisticated detonators like those the Irish Republican Army has used can give far longer lead times, up to several days.
"You wouldn't need very advanced knowledge to make one of these," Oppenheimer said.
Law enforcement officials declined to respond to questions about a U.S. official's statement that evidence indicating timers were used was found in the debris. London police also played down the possibility the devices were detonated by remote control using cell phones.
Some experts believe the bomber on the double-decker bus may have blundered, blowing up the wrong target and accidentally killing himself. Media reports have quoted an witness who got off the crowded bus just before it exploded as saying he saw an agitated man in his 20s fiddling anxiously with something in his bag.
"Everybody is standing face-to-face, and this guy kept dipping into this bag," Richard Jones, 61, of Berkshire, west of London, told the British Broadcasting Corp.
Standish said the man may have intended to leave his bomb on the subway but was unable to board because his co-conspirators already had shut the system down. He may have gotten on a bus instead and detonated the package sooner than he meant to, killing himself.
Police say there is no indication the attackers were suicide bombers, but they have not ruled out the possibility.
Al Qaeda is a different terror network now than it was in 2001, when leaders commanded a more hierarchical, well-organized collection of cells.
Those responsible for the London attacks may have been British citizens with no formal terrorism training or direct links to Al Qaeda commanders, Standish said.
"I suspect that this is a low-level, possibly locally recruited Al Qaeda cell," he said.
"Al Qaeda is now an ideology. It's moved beyond being a structural organization," he said. "All one has to do to form an Al Qaeda cell is to get together with a group of like-minded individuals and say, 'We are going to start an Al Qaeda cell.' ... If one is prepared to carry out an attack in the name of Al Qaeda, one becomes an Al Qaeda operative."
That kind of loose grouping is far harder to battle than a more tightly knit group, Standish said.
He said the bombers' choice of targets reflected a lack of knowledge about the mechanics of explosions that suggests they were not highly trained or experienced.
Bombing a tightly enclosed space like an Underground train is likely to kill fewer people than targeting a more open space where debris can fly through the air and devastate a wider area, he said. In a crowded Tube train, the primary force of a blast is likely to be absorbed by a small number of people around the explosion and by the train itself, he said.
In Baghdad, the chief government spokesman said Friday that Islamic extremists have been using Iraq as a planning center for attacks around the world since losing Afghanistan as their base in 2001.
Speaking about the London attacks, Laith Kubba said "we don't know exactly who carried out these acts but it is clear that these networks used to be in Afghanistan and now they work in Iraq."
The spokesman said that insurgents in Iraq and those who carried out the London attacks "are from the same network. There are different groups in the world, but they all follow the same school."
Britain is home to a number of known militants whom police will likely scrutinize as they seek clues to the perpetrators' identities.
Among them is Mohamed Guerbouzi, convicted in absentia in his native Morocco in 2003 and sentenced to 20 years in prison in connection with the Casablanca bombings.
French officials consider Guerbouzi, who has British and Moroccan nationality, to be the founder and principal recruiter of the Moroccan Islamic Combatant Group.
Morocco has sought his extradition but Britain has not complied, French judicial officials say.