The bombs were probably made from simple, relatively easy to obtain plastic explosives, not the higher-grade military plastics such as Semtex, which would have killed far more people, said Andy Oppenheimer, a weapons expert and a consultant 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 them, are missing from Czech stocks, he added.

Backpack size

Police said the four bombs that hit the London transportation network on Thursday weighed less than 4.5kg each, small enough to be carried in a backpack.

They were left on the floor of the Tube trains and either a seat or the floor of the No 30 bus that was ripped apart in the Bloomsbury neighbourhood, said Assistant Police Commissioner Andy Hayman.

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, 21.3m (70ft) below street level, he said.

Trapped

Bodies are still trapped in the mangled Piccadilly line train between King's Cross and Russell Square stations, the site where at least 21 people were killed.

King's Cross station remained
closed on Friday

Rescuers got all the survivors out in the first 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 Sir Ian Blair, commissioner of London's Metropolitan Police.

Oppenheimer said the bombers likely used a fairly basic timer which 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.

Bungled attempt

Some 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 eyewitness, 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.

"Everybody is standing face-to-face and this guy kept dipping into this bag"

Richard Jones,
No 30 bus passenger

Standish said the man might have intended to leave his bomb on the subway, but was unable to board because his co-conspirators had already shut the system down. He may have gotten on a bus instead and detonated the package sooner than he meant to, killing himself.

Those responsible for the London attacks may have been British citizens with no formal terrorism training or direct links to al-Qaida commanders, Standish said.

"I suspect that this is a low-level, possibly locally recruited al-Qaida cell," he said.

"Al-Qaida is now an ideology, it's moved beyond being a structural organisation," he said.

"All one has to do to form an al-Qaida cell is to get together with a group of like-minded individuals and say 'We are going to start an al-Qaida cell.' ... If one is prepared to carry out an attack in the name of al-Qaida, one becomes an al-Qaida
operative."

Splinter groups

That kind of loose grouping is far harder to battle than a more tightly knit group, Standish said.

Blasts in open areas can be far
more deadly due to flying debris

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 such as 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.