CNN is reporting that the roughly 10 ounces of explosives Reid had in his shoe were PETN-based plastic explosives:

PETN is one of the compounds that makes up plastic explosives such as Semtex, the main ingredient in the bomb that brought down Pan Am Flight 103 over Lockerbie, Scotland, in 1988.

PETN — Pentaerythrite tetranitrate — is a sophisticated explosive compound and not easily obtained, according to experts, and there are few civilian uses for it.

Well, at least something funny