As the search for missing Malaysia Airlines flight MH370 continues in the southern Indian Ocean, some key questions remain unanswered.
Here are 10 questions about what happened to the Boeing 777 that disappeared after leaving Kuala Lumpur bound for Beijing on 8 March, with 239 people on board.
1. Why did the plane make a sharp left turn?
Military radar logs show flight MH370 turned unexpectedly west when it diverted from its planned flight path, by which time the plane's transponder had already been switched off, and its last ACARS datalink transmission sent.
Sudden turns like this are "extremely rare", according to Dr Guy Gratton of Brunel University's Flight Safety Lab. He says the only real reason pilots are likely to make such a manoeuvre is if there's a serious problem on the plane which makes them decide to divert to a different destination, to get the aircraft on the ground.
That could be a fire, other aircraft in the area, or sudden decompression, according to David Barry, an expert on flight data monitoring at Cranfield University.
Malicious intent - by a pilot or intruder - is another possibility.
But unless the "black box" flight recorders are found, whatever happened in the cockpit at that moment will remain in the realms of speculation.
2. Is it reasonable to speculate that a pilot could have intended to kill himself?
There has been much speculation in the media that suicide might have been behind the loss of the plane.
It wouldn't be the first time it's happened. The crashes of Egypt Air flight 990 in 1999 and Silk Air flight 185 in 1997 are both thought to have been caused deliberately by a pilot, though the view has been contested. The Aviation Safety Network says there have been eight plane crashes linked to pilot suicide since 1976.
So far, no evidence has been released from searches of the homes of Captain Zaharie Ahmad Shah and his co-pilot Fariq Abdul Hamid that back up any similar explanation for MH370. There has been speculation that Shah may have been upset after breaking up with his wife, but there is so far no reliable source for his state of mind. It's been reported police are still examining a flight simulator found in the captain's home.
Barry says the apparent turning off of certain systems might give weight to the theory, but "pilot suicide is a theory like any other". Gratton agrees. "There simply isn't any evidence to prove or disprove it," he says.
3. Is a hijack scenario even possible?
Airliners have been fitted with strengthened flight deck doors - intended to prevent intruders from taking control - since 9/11. David Learmount, safety editor at Flight International magazine, says they are "bulletproof" and "couldn't be penetrated with an axe".
Sylvia Wrigley, light aircraft pilot and author of Why Planes Crash, agrees it's unlikely anyone would be able to force their way in. "Even if the door was being broken down, they wouldn't be able to get in before there'd been a mayday call, unless the pilots were incapacitated," she says.
However, one former pilot, who did not wish to be named, has suggested there is theoretically a way to disable the lock and get into the flight deck.
But in any case, however secure the door, there are times when the door is open - when a member of the crew either visits the toilet or has to check on something in the cabin. It's always been pointed out that it would be possible to rush the cockpit when this is the case. Some airlines, including Israel's El Al, have double doors to guard against this scenario. Gratton says there's a procedure which requires a member of the cabin crew to guard the door when it's opened.
But even in the event of hijackers rushing the cockpit, it would be easy for either crew member to send a distress signal.
The security of the cockpit door offers protection against intruders, but it also prevents action being taken if something does go wrong. Last month the co-pilot of an Ethiopian Airlines flight waited for the pilot to go to the toilet before hijacking the aircraft and flying it to Switzerland.
There's also the possibility that a pilot invited a passenger in. Photographs have emerged of the co-pilot of MH370 entertaining teenage tourists in an aircraft cockpit during a previous flight.
Boeing said it would be inappropriate to comment on an ongoing investigation.
4. Is there an accidental scenario that stands up to scrutiny?
So far most theories have been based on the assumption that the communications systems and the plane's transponder were deliberately disabled, a view endorsed by Malaysian officials.
However, Wrigley believes it's possible a sequence of events may have taken the plane so far off course by accident. "Something could have gone wrong in stages. A fire could have taken out part of the plane, or led to some systems failing, but left the plane intact. Then there could have been decompression - not an explosive decompression, but a gradual one," she says.
Wrigley cites the Helios Airways flight 522 which crashed into a mountain in Greece in 2005 after a loss of cabin pressure and lack of oxygen incapacitated the crew, but left the plane flying on autopilot, as an example. "If the Helios plane hadn't hit the mountain, it would have kept going until it ran out of fuel. I'm not saying it's a likely scenario, but it's not impossible," she says.
Pilots have pointed out that one of the very first actions in many emergency drills is to send a message to air traffic control or some other form of signal. For a purely accidental scenario to make sense, whatever initial event took place must have simultaneously knocked out all regular means to communicate with the ground.
5. Why was no action taken when the plane's transponder signal went off?
MH370's transponder - which communicates with ground radar - was shut down as the aircraft crossed from Malaysian air traffic control into Vietnamese airspace over the South China Sea.
If a plane disappeared in Europe, Barry says someone in air traffic control would have noticed and raised the alarm pretty quickly. Gratton agrees. "In Europe handover is extremely slick.
"At the very least I'd expect air traffic controllers to try and contact a nearby aircraft to try and establish direct contact. Pilots frequently use TCAS [traffic collision avoidance system], which detects transponders of other aircraft to ensure they aren't too close to each other," he adds.
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