I can think of several possibilities that would cause a catastrophic engine failure on takeoff. Only a couple of those possibilities involve terrorism.

1. A manufacturing defect caused a turbine or compressor failure. This was the cause of the Sioux City, Iowa crash in the late 1980s.

2. A maintenance failure, such as what happened with the Chicago DC-10 crash in 1979. The plane was almost an hour late for takeoff. I don't know if this was a routine delay or if the plane was having mechanical problems. If the plane was having mechical problems, it's possible they weren't fixed properly or it's possible a mechanic accidently left a tool in the engine. It happens.

3. Bird injestion, like what caused an E-3A AWACS crash in Alaska several years ago.

4. Foreign object (FOD) injestion that could've cause the engine to come apart or could've been thrown through the cowling.

5. Sabotage - can't rule that out yet. If that was the cause, it could've been done by one or more mechanics or possibly by some baggage handlers. I've heard too many reports of unscreened people working on flight ramps where they have access to planes.

6. Bomb - either near the engine or in the baggage hold. If it was in the baggage hold, pieces of the plane might've FODed the engine.

While most eyewitnesses have proven unreliable when it comes to aircraft accidents, I listen to airline pilots very carefully. If they reported the engine was smoking on takeoff, then that seems to weaken the bomb theory. It still could've been sabotage, such as intentionally damaged or loosened fuel lines or tampering with the fuel system. At the moment, I'm leaning more to poor maintenance or some other "natural" cause.