The disappearance of Malaysia Airlines Flight 370 is so baffling that veteran aviation investigators say they are struggling to come up with a plausible theory in the absence of any wreckage.
There are only a handful of scenarios that could explain how a usually reliable wide-body jet could seemingly vanish from a clear sky with no distress call and no obvious debris field.
And for every circumstance that would seem to support one theory, another undercuts it.
"It's a thousand-piece puzzle, and we have two pieces and we're trying to make a picture with that," said John Goglia, a former member of the National Transportation Safety Board.
"I'm totally confused, to be honest with you. Nothing makes sense."
Added former NTSB investigator Greg Feith, "You can't rule anything out at this point."
Here are the possibilities investigators are likely looking at as they try to figure out what happened to the Boeing 777 flying 239 people from Malaysia to Beijing across the South China Sea.
Midair mechanical malfunction
A catastrophic event that made the jet come apart at its cruising altitude of 35,000 feet would explain its sudden disappearance. But Feith said that would result in a large debris field in the water, and no one has found one yet.
On the other hand, a mechanical malfunction that brought the plane down intact presumably would have given the pilots time to make a distress call. Even if there were a total electrical power failure, backup systems would have kept the radio running.
Bomb or other explosion
Again, if there were a cataclysm over water — where authorities say the plane lost contact and where the search is concentrated — debris would likely have been spotted already.
"Lots of things inside an airplane float. People float, to some extent," Goglia said.
What if the blast were smaller? Feith can conceive of a situation in which a bomb blew open a hole big enough to cause explosive decompression but didn't damage the structural integrity of the plane. The fuselage would fill with fog, and passengers and crew would pass out within five to 15 seconds.
On autopilot, as the plane would be at that point of the flight, it could fly for hours before crashing on land, perhaps in a thick, remote jungle, Feith said.
Weighing against a bomb is the fact that no one has claimed responsibility, and a plane carrying mostly Malaysian and Chinese passengers would seem to be a less likely terrorist target than an American or European jet.
Pentagon surveillance data also uncovered no sign of a midair explosion.
Increased security has made skyjackings largely a crime of the past. Flight 370 would have had a cockpit door, and procedure would have called for it to have been locked at that point in the trip.
Nevertheless, it's impossible to tell what security measures were actually in place on the flight. We already know that Malaysia didn't bother to check whether the stolen passports used by two passengers were listed in an Interpol database.
If an armed person did manage to get into the cockpit, he or she might prevent the crew from making a distress call. There are discreet codes for a hijacking that pilots can put into the transponder, but because radar coverage in that area was unreliable, it might have gone unnoticed, Feith said.
Typically, hijackers take over a plane to land it somewhere other than its intended destination or to make a demand of some sort — neither of which appears to fit with this case. However, as 9/11 showed, a suicidal hijacker could wrest away control of an aircraft for the purpose of terrorism and intentionally or accidentally crash it into anything.
Pilot error or sabotage
The Boeing 777 should have been on autopilot, but let's say the pilots took it off and then got disoriented or made some other mistake. They still should have had time to make an emergency call once they got into trouble.
In the 1999 crash of EgyptAir Flight 990, authorities believe co-pilot Gamil El Batouty directed the plane into the Atlantic Ocean off Nantucket, Mass., while the pilot desperately tried to regain control.
Experts say that theoretically could have happened with the Malaysia Airlines flight — although there is no evidence to suggest it did.
Bringing an intact plane straight down into the water would explain the lack of a sprawling wreckage field. If there were a struggle in the cockpit, that might be a reason no distress call was made.