Unlike Czech-made Semtex, the more popular choice among terrorists, C-4 is an expensive and relatively hard to get explosive, yet the suicide bombers used as much as 600 pounds of it.
"It's not an explosive that is available on the market except for highly sophisticated, organized groups capable of getting it from certain governments
or states," said Yemeni Prime Minister Dr. Abdul Karim al-Iryani.
Developed in the '50s
Every bomb squad in the United States is familiar with C-4, which was developed in this country in the 1950s.
"A pound and a quarter can destroy a vehicle, can take out a room in a house or something of that nature," said Montgomery County, Maryland, Assistant District Fire Chief Brian Geraci.
C-4 -- short for Composite-4 -- is a mix of a material called RDX (Research Development Explosive) and a plasticizer that gives the material a firm but pliable form like putty. It can be pushed into any shape and has a long shelf life.
U.S. officials believe C-4 was used in 1996 to blow up the Khobar Towers, killing 19 U.S. service personnel in the military housing complex in Saudi Arabia.
The use of C-4 against the USS Cole suggests to some bomb experts that a government hostile to the U.S., such as Iran or Iraq, may have been the original source of the material. However, C-4 is widely distributed around the world.
Many U.S. allies have it and experts say a similar formula has also been
produced in Canada, Austria and possibly Iran.
In the 1970s, renegade ex-CIA agent Edwin Wilson was convicted of shipping 21 tons of C-4 to Libya for use in what the U.S. government said was a school for terrorists that he set up there.
Explosive residue tells on its maker
Experts say careful lab work may determine exactly where the C-4 used
against the Cole came from if technicians can identify traces of other chemicals in the explosive.
The United States and many other countries have signed a pact mandating that plastic explosives be tagged with selected chemical marking agents to facilitate their detection.
That agreement was negotiated in the wake of the 1988 Pan Am 103 bombing and is aimed at combating the use of plastic explosives by terrorists.
"Each (C-4) factory will have something of a signature, based on the other things that are produced there, that will be contaminants in the explosive residue from the Cole," said explosives expert Jack McGeorge.
U.S. officials hope with patient police work to find the C-4's origin and then trace how the explosive ended up in the hands of the suicide bombers who attacked the Cole.
CNN National Security Correspondent David Ensor contributed to this report.
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