Even a small amount of C-4 — the explosive that may have been in the shoe of a passenger aboard a diverted jetliner — could have been enough to destroy the plane, an expert said Saturday.

But it would have been hard to set off.

An ounce of the plastic explosive could have been enough to blow out a window or wall of an airliner at altitude, and then "the air pressure would rip the plane apart," said Jack O'Keefe, a bomb technician with the Boston police bomb squad.

However, O'Keefe said he doubted that the C-4 could have been set off without a blasting cap, and walking around with one would have been very dangerous for the passenger.

The explosive was used in the October 2000 bombing of the USS Cole in Yemen, killing 17 U.S. sailors and wounding 39.

In October, a third of a pound of C-4 was found in an abandoned suitcase inside a Philadelphia bus terminal. Police said that was enough explosive to level the building.

Used extensively during the Vietnam War, C-4 is prized because it is easy to shape and relatively hard to set off by accident. C-4's main ingredient is RDX, which is also used in fireworks. The puttylike substance can be easily molded by hand. Its shape can dictate the force and direction of its blast.

It is relatively insensitive to impact, friction or fire, although large quantities can explode if burned.

For smaller amounts, "if you set it on fire, it'll just burn, unless you hit it with something," O'Keefe said.

That made it popular with American soldiers in Vietnam, who would break off a small piece to quickly heat water or C-rations.

"It wouldn't go off. It's very safe," O'Keefe said. "That's why the military uses it. It could take a bullet from a high-powered rifle without detonating."

C-4 does not deteriorate over time but can crumble in cold weather, O'Keefe said.

Occasional thefts of military C-4 have been reported, and O'Keefe said some Vietnam War veterans kept small amounts as souvenirs.

C-4 recipes are available via the Internet and in books. Amazon.com, the giant Web book merchant, offers used copies of an out-of-print book called "Homemade C-4 : A Recipe for Survival."

But cooking up the explosive at home is a tricky and dangerous process, O'Keefe said.