While nothing can yet be ruled out, what is known about the erratic behaviour of EgyptAir Flight MS804 before it crashed suggests the cause was human rather than technical, or potentially a combination of both, aviation experts have said.
Greek authorities say the plane swerved 90 degrees left and then 360 degrees right before it plummeted into the Mediterranean Sea.
Here are the main scenarios presented by experts based on that erratic flight path:
Cockpit struggle
The swerving suggests some kind of struggle inside the cockpit, said Philip Baum, editor of Aviation Security International.
He said the pilots could have been trying to control an aircraft disabled by an explosion, like in 1976 when two bombs exploded on a Cuban passenger plane after take-off from Barbados and the pilot tried to steer the aircraft away from a beach.
Or they could have been struggling with someone trying to take control of the plane.
“It could have been a fight in the flight deck between crew members, one suicidal and one not. Or a hijacker trying to gain access.”
In 2000, British Airways Flight 2069 from London to Nairobi nosedived and dropped 10,000ft after a deranged passenger burst into the cockpit and grabbed the flight controls.
He was overpowered and the flight crew stabilised the plane.
The Egyptian military said no distress call was received from the pilot in the EgyptAir crash.
If there was a struggle over the flight controls, that would be understandable, Mr Baum said.
“The last thing you are thinking about when you are struggling is to send out a distress signal. The first thing you think about is trying to regain control of the aircraft.”
Sudden impact
Another possibility is that the plane was hit by an external object that knocked it out of the sky, said Philip Butterworth-Hayes, an aviation systems expert.
“It could have been hit by a missile or a drone. Something hits it and changes the course.”
Hans Kjall, of the Nordic Safety Analysis Group in Sweden, called that scenario "relatively unlikely".
He said the plane’s position over the Mediterranean Sea means a missile strike would have required sophisticated military weapons systems.
“You would need a seaborne missile.”
He said that if there was an attack on the plane, it was more likely that it happened inside the aircraft, such as an “act of terrorism”.
Technical failure
All experts say it is too early to rule anything out, but Mr Butterworth-Hayes said it is difficult to imagine that a technical mishap caused the crash.
“I can’t think of a technical fault. Because you have three flight control systems. And even if they all fail, a pilot can still fly the aircraft, they can keep it straight and level.”
Mr Kjall said that if the plane went down due to some kind of systems failure it was probably in combination with the human factor.
That scenario can happen if the navigation systems feed “erroneous information to the cockpit, fooling the pilots into making wrongful manoeuvres”, he said.
The most prominent example of a mid-flight crash linked to systems failures was Air France Flight 447, which disappeared over the Atlantic Ocean during a flight from Brazil to France in 2009.
A storm, faulty data and human error all played a part.
David Learmount, consulting editor at Flight Global, said one similarity with the Air France crash is “they both happened in the middle of the night”.
“It is when human beings are at their lowest-possible performance level. Whatever happens, the pilots would not be as bright as they would have been had it been in the middle of the day.”
PA