It could have been a fire, a bomb, a cascading electrical failure - or something else altogether.
The first hard evidence about what brought down EgyptAir Flight 804 last week - a string of seven error messages sent automatically minutes before the Airbus A320 plunged into the Mediterranean - has done little to narrow down what happened, according to aircraft specialists and accident investigators.
"Something's wrong. We know that," Michael Barr, an accident investigation instructor at the University of Southern California, said. "Until they get the data recorder back, it would be hard to come up with an idea."
An Egyptian forensics investigator said today that those body parts retrieved from the sea so far have been small. That might suggest some sort of explosion, he said, though bodies can also be ripped apart when an aircraft disintegrates following a structural failure, or hits the ground or sea at high velocity.
Uncertainty over what caused the May 19 crash is delaying any response by policy makers, whether it be a hunt for a terrorist who planted a bomb, or revising safety procedures to prevent similar types of accidents in the future. The outcome of the investigation could bring starkly different results.
1 Indications of fire
The error messages that emerged over the weekend include two separate alarms indicating smoke, suggesting there could have been a fire before the flight from Paris to Cairo plunged into the water, killing all 66 aboard. One of the alarms reported that smoke had been detected in the compartment directly beneath the cockpit where the plane's computers and avionics equipment are located. The other was in a toilet.
Other alerts showed unspecified problems with flight computers, including one that pilots use to set flight parameters such as altitude, and another that's part of the Airbus's complex flight-control system. Both computers were located in the electronics bay where smoke was detected.
The error messages were confirmed by the French accident investigation agency BEA over the weekend after they were first reported on the Aviation Herald website. Three additional error messages were sent about windows on the right side of the cockpit, where the co-pilot sits.
Two people who are familiar with the Airbus A320 design said while the data may point to a fire on the plane, it didn't rule out other possibilities.
Some type of bomb or incendiary device might have triggered the alerts, they said. A widespread electrical failure was also conceivable. At this early stage in the investigation, they said, the cryptic alerts don't rule out much.
2 Timing of alerts
The timing of the alerts is also puzzling. They began at 2.26 am local time and continued for three minutes. That coincides with the plane's last transmission indicating its position, which came at 2.29:33 am, according to the online flight tracker FlightRadar24.
In previous cases in which a bomb or explosion destroyed an airliner, the electrical systems failed almost instantly instead of over several minutes.
By contrast, fires that have led to crashes on large planes have typically taken much longer to take them down. A United Parcel Service Inc. jumbo jet that crashed near Dubai in 2010 after a blaze erupted in the cargo hold flew for more than 27 minutes before it slammed into the ground, according to the General Civil Aviation Authority of the United Arab Emirates.
Even in cases in which fires moved more quickly, such as the 1996 crash of a ValuJet plane near Miami, pilots were able to make radio calls alerting air-traffic controllers to the crisis and attempt an emergency landing.
The first sign of a fire on the ValuJet flight occurred a few minutes after takeoff, according to a report by the US National Transportation Safety Board.
"We got some electrical problem," the captain said, according to a transcript of a cockpit recording. "We're losing everything."
Investigators later determined that a ferocious fire fed by pure oxygen had erupted in the cargo hold near the front of the plane. Within seconds, shouts of "fire" could be heard in the cockpit and the co-pilot was radioing a controller to request an emergency landing.
Within just over three minutes, the pilots could no longer control the plane as the fire spread and it dove into a swamp, according to the NTSB.
In the EgyptAir crash, there were no such radio calls from pilots, according to Ehab Azmy, the head of Egypt's National Air Navigation Services. Azmy denied a report by French television channel M6, which said pilots had discussed a fire with Egyptian controllers.
One area investigators will want to examine is the emergency oxygen supply for pilots, which is located beneath the cockpit roughly where smoke was detected and other errors were occurring, according to the people familiar with the A320.
3 Automated flight-control system
Another topic investigators may examine is the plane's automated flight-control system, they said. Unlike more traditional aircraft, pilot controls are transmitted to a group of computers, which then command flight panels to move to steer the plane.
If a fire destroyed those computers, the plane would be difficult to fly, they said. However, the Airbus is designed to stay aloft even if all its computers fail, so it should have been possible to continue flying.
None of errors reported by the plane by themselves should have led to a crash, they said. Reaching a more definitive conclusion depends on whatever additional evidence turns up in the search for the plane's wreckage.
Debris from the plane has been found about 290km off the Egyptian coast and the Egyptian Government has sent a submarine to the scene in an attempt to locate wreckage, particularly the two crash-proof recorders. One captures sounds in the cockpit and the other tracks hundreds of parameters indicating how the plane is flying.
Until more concrete data from those recorders can be analysed, it doesn't make sense to pick one theory over another, according to Tony Fazio, who headed the US Federal Aviation Administration's accident investigation office for five years before starting a consulting company.
"That's always our bottom line," Fazio said. "Anybody with any credibility will tell you that it's just way too soon."