http://www.azcentral.com/news/natio...a-missing-jetliner-clues-possible-causes.html
Hijacking. A traditional hijacking seems unlikely given that a planes captors typically land at an airport and have some type of demand. But a 9/11-like hijacking is possible, with terrorists forcing the plane into the ocean.
Pilot suicide. There were two large jet crashes in the late 1990s that investigators suspected were caused by pilots deliberately crashing the planes.
Accidental shoot-down by some countrys military. In July 1988, the United States Navy missile cruiser USS Vincennes accidently shot down an Iran Air flight, killing all 290 passengers and crew. In September 1983, a Korean Air Lines flight was shot down by a Russian fighter jet.