What could have happened
* Structural failure of airframe or engines
But given the planes long history and impressive safety record, experts suggest this is unlikely.
* Rupture due to constant pressurisation and depressurisation of cabin for takeoff and landing
But such a rupture is not likely. The 777 is usually flown on longer distances, with many fewer takeoffs and landings, putting less stress on the airframe.
But all indications show that there were clear skies.
The pilots could have taken the plane off autopilot and somehow went off course and didnt realise it until it was too late. But the plane probably would have been picked up by radar somewhere.
Failure of both engines.
But sources said the plane could glide for up to 20 minutes, giving pilots plenty of time to make an emergency call.
* Deliberate pilot suicide
* Accidental shoot-down by some countrys military