Tracing source of the explosives may reveal connection to al-Qa'ida

The discovery of a substantial quantity of explosives at the so-called "operational" base of the bombers in a Leeds suburb has reinforced suspicions the men were linked to a wider network, capable of obtaining such material. Tracing the source of the explosive and how it reached Leeds will help police ascertain whether an al-Qa'ida '"guiding hand" or bomb-maker was involved with the group. "Those people who died were clearly expendable. What the police are searching for now are what are termed the 'LBAs' - left to bomb again," said one terrorism expert yesterday.

Samples of the explosives from all the bomb scenes, together with those from Leeds and the car found at Luton were undergoing detailed forensic examination for clues as to their origin.

The material can be compared with databases of commercial and military explosives and those used in other Islamic terrorist incidents around the world.

Charles Clarke, the Home Secretary, yesterday denied reports that the explosive was believed to have come from the Balkans or Eastern Europe. Terrorism experts said that region was just one of many where Islamic extremists operate and which could have been the source.

Andy Oppenheimer, an expert on terrorism and explosives for Jane's Defence Weekly said: "While the Balkans is a prime area as a black market source, there remains the possibility it might also have come from Iraq or even the Chechens."

The explosive is most likely to have been smuggled into Britain through a sea route rather than an airport. It could have been shipped in a sealed container or in the framework of a vehicle to avoid explosive sniffing machines or dogs.

Military explosives are favoured by terrorists because they are stable, easy to handle and easily available on the blackmarket.

The most notorious is Czech-made Semtex, of which the IRA obtained large quantities during the 1980s. "It is the ultimate explosive of choice," Mr Oppenheimer said.

"A very small amount can cause a very large explosion." The fact that the London bombs were said to be up to 10lb each in weight tends to discount Semtex.

Other possible varieties include the American made C4 - used in other al-Qa'ida-linked attacks such as the bombings in Bali in October 2002 and HMX, the type stolen in Iraq last October. Mr Oppenheimer stressed it was just as likely that the bombers could have used ordinary explosives obtained from somewhere in Britain - such as a building site or quarry.

Although a trained bomb-maker could have been involved, instructions for making bombs are readily available on the internet and since the bombs were most likely to have been self-detonated, there would have been little need for complicated timers.