What happened to Malaysia Airlines Flight 370, which vanished Saturday over the South China Sea en route to Beijing? Former U.S. transportation officials are stumped. Here are some key theories that investigators are considering as they struggle to understand how a usually reliable wide-body jet could apparently disappear from a clear sky — with no distress call and no apparent debris field, either. A mid-air mechanical malfunction would likely have caused either a debris field or allowed time for a distress call. An explosion would likely leave debris, too, and a bomb is unlikely given the target. Maybe a hijacking? That seems unlikely given security — and what would have happened to the plane thereafter? Pilot error or sabotage could also be possible, in theory — but an emergency call should still have been possible in that case. Click through to read more about some of the possible fates experts are considering for the missing plane.