Semtex-style C4 explosive 'widely used by al Qaeda'

Although C4, the explosive which could have been used by the mid-air attacker, is one of the most potent and dangerous explosives known, it is surprisingly available.

For anyone with an End User Certificate - a licence granted to legitimate operators such as mining companies and quarries - it can be bought from various manufacturers and is even sold over the internet. This wide availability leads to the explosive finding its way on to the black market, but for terrorists there are other sources.

The C4 plastic explosive is of the same type as Semtex, a favourite of the IRA and other terrorists. The IRA is believed to have three tons of Semtex and the material is often traded between clandestine organisations across the world.

The training manuals of Osama bin Laden's al Qaeda network go into detail on how it is used most effectively and even how it can be manufactured. The material has two main components: RDX, or Cyclonite, the explosive similar to that found in fireworks, and PETN, or Pentaerythrite. When the two are combined the mix turns from a powder to a putty-like substance.

In the tribal lands of Pakistan, on the Afghanistan border, plastic explosive is sold over the counter at a number of outlets specialising in the cash sale of weapons and military equipment. Most of this material is made locally in workshops which turn out everything from copied AK-47 assault rifles to artillery pieces. A favourite is the shoulder-fired rocket launcher which uses rounds loaded with C4.

The explosive can be easily smuggled in small quantities: it is virtually odourless and only highly trained sniffer dogs are effective in detecting it.

French authorities originally thought the suspected bomber on the American Airlines plane was a Sri Lankan national, and if he proves to have a Sri Lankan connection he may have obtained explosives in a part of the country controlled by Tamil Tiger guerrillas where military material is available.

Terrorist expert Mike Yardley said: "If the stuff was genuine C4 then he probably has an al Qaeda background.

"But there has to be a suspicion that this was a less sophisticated substance and if that proves to be the case then he is probably a lone madman."