Twenty-one days after it disappeared en route from Kuala Lumpur to Beijing, there are still few facts that point to what exactly happened to Flight MH370.
Based on these, experts have three theories, though there may be more as clues emerge.
It is known that the jet's communication and tracking systems - the transponder, and the Aircraft Communications Addressing and Reporting System or ACARS - were turned off deliberately.
This happened as the Boeing 777-200ER was leaving Malaysian airspace and entering Vietnamese territory. Shortly after, it diverted from its original flight path.
The last satellite transmission was at 8.11am on March 8 - 6½ hours after Malaysian air traffic control lost contact with the jet. Until that point, there was no distress signal from the cockpit.
On Monday night, Malaysian Prime Minister Najib Razak confirmed, based on analysis of satellite data, that the plane had plunged into the Indian Ocean with zero chance of survivors.
No debris has been found.
The development ruled out several theories about MH370's disappearance and the fate of the 239 who were on board.
A major catastrophic mechanical failure is out. This would have caused a mid-air explosion which would have been detected.
A rapid fall in cabin pressure or in-flight fire is unlikely.
Even if there is a total engine failure at 35,000ft, a plane can continue to glide and pilots have about 15 to 20 minutes to do what they need to, such as send alerts.
In 1998, Swissair Flight 111 crashed into the Atlantic Ocean after smoke filled the cockpit and cabin. But not before the pilots reported the emergency.
Seven years later, a Helios Airways plane crashed into a mountain when a lack of oxygen incapacitated the crew. Again, the Greek carrier's pilots had time to send out a distress signal.
After eliminating all implausible theories, experts are now left with three theories that match the passage of events with MH370.
It was a botched hijack where someone on the plane, or a pilot, took control but did not achieve his intention.
Such an incident occurred in 1996 when the captain of Ethiopian Airlines Flight 961 tried in vain to stop three hijackers seeking asylum in Australia while en route from Addis Ababa to Nairobi.
The plane crash-landed in the Indian Ocean near the Comoros Islands when fuel ran out. Of the 175 passengers and crew on board, 125 died - including the hijackers.
Malaysian authorities have said they did not receive any demands over MH370, but it is possible they were in negotiations with hijackers. Some questioned if the authorities or those on board could have tried talking with the hijackers, but the plane might have run out of fuel and crashed before a decision was made.
It is also possible the pilots or crew tried to stop the criminals. A fight could have broken out and the plane crashed.
Malaysia Airlines has confirmed that the co-pilot was the one who said "All right. Goodnight", just before the transponder was deactivated.
Initial investigations showed that Mr Fariq Abdul Hamid, 27, sounded calm at the time, suggesting he was not under any threat.
Fingers have also been pointed at Captain Zaharie Ahmad Shah, 53. Police seized a homemade flight simulator from his home which experts are analysing. They are probing whether the pilots had financial or other troubles.
In the 1997 crash of SilkAir Flight MI185, investigators concluded that pilot suicide, though not proven, could have been to blame. The plane nosedived into the Musi River in Palembang, killing all 104 passengers and crew.
It is possible that something happened which knocked out everyone on the plane - without any criminal intent - causing it to fly of its own accord.
Experts say it could have taken less than a minute for the pilots, passengers and crew to lose consciousness during a catastrophic loss of pressure that somehow also led to a systems malfunction.
The authorities believe that after the plane lost contact with air traffic controllers, it could have climbed as high as 45,000ft - above the normal cruising altitude - which would have led to such a drastic fall in cabin pressure.
In 1999, six people on a business jet from Orlando to Dallas - including American golfer Payne Stewart - died after a sudden loss of cabin pressure deprived them of oxygen. The plane flew on auto-pilot for four hours before running out of fuel and crashing into a field in South Dakota.
There is another explanation for MH370, aviation and security experts said - which contains elements of the earlier three.
It cannot be ruled out that something or someone was on the plane that was important enough for an organisation or group to stage a hostile takeover.
Hijackers could have taken the plane to 45,000ft to knock everyone out, then landed it in an unknown location. After getting what they wanted, they got the jet back in the air by remote flying and crashed it.
How fast MH370 fell and whether the aircraft broke before it plunged into the Indian Ocean or on impact, depends on which of the above theories you believe, aviation experts said. This in turn determines what sort of debris could be found and whether there might even have been survivors.
If it was pilot suicide, it is likely the plane nosedived steeply causing an impact which would have caused the aircraft to disintegrate into small pieces even before it hit the waters.
If a plane crashes because fuel runs out, it would plunge into the ocean "within a few minutes", said Mr H.R. Mohandas, a former pilot who teaches aviation at Republic Polytechnic. Such an impact would be akin to hitting concrete and cause debris from the plane to scatter over a large area.
If there was someone flying the aircraft, the descent would have taken about 15 minutes before it hit the water, he said. While the impact could have left some survivors, they are unlikely to have survived long in the icy cold waters.
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