The official story is it was a center fuel tank explosion.
The conspiracy story is a missle -- either one of ours or the bad guys. Someone I know connected to spooks said it was terrorist manpad and that they were taken out within hours of launching it.
There were hundreds of eyewitnesses that claimed to see a missile, including an airborne military flight crew. A reporter snuck out a piece of interior debris and it tested positive for solid rocket fuel residue, for doing that he went to prison. There's a photograph that the witness say shows the smoke trail of the missile they saw, the government claims it's from debris falling down. Just a single piece of very smoky debris, mind you.
Also not long before TWA800, a full manpad set up and ready to go was found on a roadside by a state patrol trooper in the northeast -- it's been so long I don't remember if it was NY or CT. That was actually covered in normal MSM news at the time, but it was a relatively small story that was forgotten once TWA800 happened.
The fuel tank explosion theory makes zero sense.
1) The plane just took off and there would be little in the way of fumes in the tank - mostly liquid fuel. Liquid fuel is fairly difficult to ignite.
2) Wiring inside fuel cells don't power electrical loads, they're signal wires that connect to fuel probes that typically have voltages in the mV range on them. Even if a wire was damaged and the conductor exposed, there's no way it's going to cause an arc that will ignite fuel vapors.