AMERICAN analysis of residues found in the wreckage indicates that the bombers used C-4, a military plastic explosive which has no non-military uses and is not available on the open market..
To some experts, this suggests the involvement of a state, or at least a well-organised group. C-4 was developed for the US in the Vietnam era. It has been sold by the US to Saudi Arabia, Kuwait and Iran (under the Shah), and several Nato countries possess it. The US also used it in the 1991 Gulf war.
The formula for C-4 is not secret, and quantities have occasionally been stolen. About 20 years ago, a former CIA agent was convicted of shipping 21 tons of C-4 to Libya - allegedly for terrorist training.
C-4 does not deteriorate with age, so the explosives used in the Aden bomb could, conceivably, have been stolen at any time since the Vietnam war.
It is possible that further analysis may indicate where the explosive was manufactured and thus open up a line of investigation into how the bombers obtained it.
It is, perhaps, worth noting that the 1998 bombings of the US embassies in Tanzania and Kenya, for which Osama bin Laden has been blamed, did not use C-4 explosives, though they used detonators containing the C-4 component, RDX.
It is thought that the bombers used 400-700 pounds of explosives. This is a large amount to conceal aboard an inflatable. Although no details of the size and type of craft used have emerged so far, the bombers seem to have had trouble keeping it afloat during a test run (or previous bombing attempt) in January.
The choice of C-4 indicates that the bombers had a reasonable level of expertise, because ordinary or home-made explosives would have been less effective. But it takes no more than a quick search of the internet to discover that if you want to blast a hole in metal - tanks, ships, etc - C-4 is the explosive to use.
The shape of the USS Cole, with its sides bending outwards, and pictures of the damage, show that the force of the blast was directed both sideways and upwards. It was not the sort of attack that is expected in modern warfare - which may also help to explain the extent of the damage.
According to Paul Beaver, of Jane's Defence Weekly, the ship was "designed to withstand saturation attacks by Russian aircraft and all sorts of things," but "not designed for asymmetrical warfare … it's not what people expect these days."
Yemeni sources say the attack on the USS Cole was not the first attempt to blow up an American warship in Aden harbour. An attack on the American destroyer, USS Sullivans, in January 2000 had to be abandoned because the attackers' boat almost sank under the weight of explosives (AP 11 November, CNN 12 November). It appears that as a result of this the bombers called on an unnamed foreign expert for advice, and that the expert may have helped to shape the charge used against the Cole, maximising its effect.