For the conspiracy theorists - think about this.
SAMs very rarely cause an aeroplane to explode into lots of little bits the way that TWA 800 did. The typical SAM (or indeed air to air missile) relies upon a proximity fuse and warhead fragmentation and schrapnel to slice off big chunks of the airframe, causing the aeroplane (or what is left of it) to fall out of the sky. The result is that the debris is generally found to be in large pieces.
TWA 800 disintegrated in lots of little bits - the debris being much more compatible with an on-board rather than external explosion. So unless a bomb was on board the aeroplane then it was some part of the aeroplane itself (i.e. centre fuel tank pumps and associated vapour) which caused the explosion in the first place.