11.   Instead, he said, three theories are being investigated, a bomb, a missile attack and a catastrophic mechanical malfunction.

12.   Lebow said that the aging equipment did not cause the derailment and that there was no evidence of a mechanical malfunction on the train.

13.   Mechanical malfunction?

14.   Nonetheless, investigators have yet to come up with a plausible theory for a deadly mechanical malfunction.

15.   Officials continue to say only that the plane was downed by a catastrophic event that was either caused by a mechanical malfunction, a missile, or a bomb.

16.   Others say there is a growing focus on mechanical malfunction.

17.   Recently, NTSB investigators said they believe the center fuel tank most likely exploded because of a mechanical malfunction.

18.   Several investigators on the terrorism panel agreed that a mechanical malfunction was still the least likely scenario.

19.   Speculation centers on mechanical malfunction, a bomb, or a missile.

20.   Specifically, the NTSB is narrowing in on theories that either faulty wiring or other mechanical malfunctions ignited the tank.