A slow or sudden decompression, causing a loss of oxygen, could have killed everyone on board.
If oxygen levels dropped, a loud, automated warning would have alerted the pilots to put on their oxygen masks and immediately descend below 10,000 feet, where there is enough oxygen to breathe without aid.
If the plane depressurized and killed its occupants, which happened on golfer Payne Stewart's business jet in 1999, that would explain the silence from crew and passengers.
But aviation experts say in that case, the plane should have kept flying automatically toward Beijing and been visible on radar.
