I strongly suspect a SUCCESSFUL detonator and a similar amount of PETN. Just like richard reede, the shoe bomber (well, almost).
The initial reports on Flt 800 actually identified the explosive as PETN and said that a device had been detonated over the empty fuel tank, thereby making the vapors explode. About a week later, those stories had disappeared and suddenly the fuel tank had just decided to blow up all by its lonesome.