Investigation - the bombs: The force and nature of the London explosions suggest that the perpetrators acquired either military or high-quality commercial explosives, possibly from eastern Europe, a leading British expert said yesterday.
Prof Hans Michels of Imperial College, London, said that, although the bombs appeared to weigh no more than 4kg or 5kg, the power of the explosions pointed to the use of high-explosive material.
Suggestions that the bombs were crude home-made devices were dispelled yesterday by Sir Ian Blair, the Metropolitan Police commissioner leading the investigation.
The source of the materials was more likely to have been continental Europe than Britain, Prof Michels said.
Commercial use of explosives is low because the building trade now uses alternative materials, while licences to use explosives must be issued by the Health and Safety Executive.
Military use of explosives is not disclosed in official documents, and there is no central UK database to monitor overall quantities of explosives. "It's quite a nebulous situation," said Prof Michels.
Countries such as the Netherlands and Switzerland have a better grasp of the quantity of explosive material within their borders. In the Balkans and eastern Europe enormous quantities of explosives have been traded on the black market.
"The problem with explosives is some of it can be detected by sniffer dogs, but large amounts cannot. X-ray equipment cannot distinguish between them and other materials," Prof Michels added. "You can't check whether it's in lorries."
When he heard about the bus bombing he thought a bomb of 4kg or 5kg was just about capable of blowing off the roof of a bus.
"But when I saw the debris and the way in which blood and human tissue was thrown on to a building from the other side of the street, it was clear it was a very powerful explosion."