The sudden disappearance of a Malaysia Airlines MASM.KL jetliner with 239 people aboard represents one of the rarest kinds of aviation disaster, and the mystery is compounded by uncertainty about which country's jurisdiction the plane came down in.
Take-off and, in particular, final approach and landing are the most inherently hazardous parts of a flight, and the periods when most accidents occur.
Saturday's red-eye flight, by contrast, vanished at cruising altitude in clear skies en route from Kuala Lumpur to Beijing. No distress signal appears to have been sent, no wreckage has been found and no aircraft malfunction has been identified.
But it has been more than 24 hours since the plane went missing and Malaysia Airlines said it was "fearing the worst".
Based on the last contact with the pilots, the Boeing (BA.N) 777-200ER is presumed to have crashed off the coast of Vietnam.
Two passengers aboard were possibly travelling under stolen passports, raising questions about foul play, but no group has claimed responsibility.
"Aircraft do not crash while en route like this," said Paul Hayes, Director of Safety at Flightglobal Ascend, a British-based aviation consultancy. "It is an extremely unusual event."
Only one other recent disaster was similar: the loss of Air France Flight 447, which crashed in the Atlantic Ocean in 2009 en route from Rio de Janeiro to Paris.
GRAPHIC - Accidents by stage of flight link.reuters.com/kux47v
The incident is likely to rekindle a debate about whether black box flight recorders should be replaced with satellite-based systems capable of sending back telemetry in real time. Such systems exist, but have so far been ruled out on the basis of cost and logistics.
In the meantime, it is unclear who will take the lead in unravelling what happened to the Malaysian airliner.
Under International Civil Aviation Organization rules, the government of the territory where the crash occurred typically has jurisdiction over the wreckage and leads the investigation. So it is likely no authority can take charge until the wreckage is found.
In this case, it is likely to be Vietnam. But if the plane went down in international waters, then Malaysia would to have control, and the United States would be involved because the plane was U.S.-built.
The lead investigator could ask the United States or another country with deep investigative abilities to take a larger role. But release of information and findings about the crash would likely remain under the control of the lead investigator nation.
U.S. regulators would have "no ability to release factual information as it would if a crash happened in the U.S.", said Kelly Nantel, a spokesman for the National Transportation Safety Board, which investigates U.S. crashes. "All of our work would be in support of the lead agency."
Malaysia Airlines Flight 370 disappeared about an hour into a scheduled flight to Beijing, with theories ranging from a sudden stall, an incident on board which caused a complete electrical failure to some kind of freak accident.
The 11-year-old Boeing took off at 12.40 a.m. (1640 GMT Friday) from Kuala Lumpur International Airport and was last heard from at 1:30 a.m., according to the airline.
The plane last had contact with air traffic controllers 120 nautical miles off the east coast of the Malaysian town of Kota Bharu. Flight tracking website flightaware.com showed the plane flew northeast over Malaysia after take-off and climbed to an altitude of 35,000 feet.
A massive search operation was under way in the seas between Malaysia and the southern tip of Vietnam, centred on the area where the plane last made contact.
Pilots and aviation experts said an explosion on board appeared to be the likely cause of the disaster. The plane was at cruising altitude, the safest phase of flight, and likely would have been on autopilot.
"It was either an explosion, lightning strike or severe decompression," said a former Malaysia Airlines pilot. "The 777 can fly after a lightning strike and even severe decompression. But with an explosion, there is no chance. It is over."
An extreme, sudden loss of cabin pressure could have caused an explosive decompression and broken the plane apart, said John Goglia, a former board member of the National Transportation Safety Board. Such a decompression can be caused by corrosion or metal fatigue in the airframe.
ECHOES OF AIR FRANCE DISASTER
The disaster is most similar to the mysterious disappearance of Air France Flight 447, which killed all 228 people on board.
Investigations were unable to conclusively come up with a reason for the crash of the Airbus A330 until the plane's black boxes - its flight and voice data recorders - were recovered from the bottom of the ocean two years later.
That recovery is likely to help investigators of the Malaysia Airlines crash.
"Recovering wreckage in the ocean is always problematic, but the industry has learned a lot from the Air France crash," said Hayes, of Flightglobal Ascend.
Experts also stressed that the Air France flight provided a cautionary tale against premature speculation.
That accident was initially blamed by the airline on a thunderstorm. Later, investigators pinpointed ice that caused faulty speed sensor readings on the plane.
But data recovered after a two-year search led authorities to conclude that pilot error had also played a part - the crew's handling of the plane after the auto-pilot was disengaged put it into a stall from which it could not recover.
(Editing by Alwyn Scott, Raju Gopalakrishnan and Alex Richardson)