Smoke was detected inside the cabin of the EgyptAir passenger plane before it crashed in the Mediterranean on Thursday, investigators have confirmed.
Smoke detectors went off in the toilet and the aircraft's electrics, minutes before the signal was lost, the Aviation Herald had earlier reported.
A spokesman for French investigators said it was too early to say what caused the crash.
Flight MS804 was en route from Paris to Cairo with 66 people on board.
The Aviation Herald said it had received flight data filed through the Aircraft Communications Addressing and Reporting System (ACARS) from three independent channels.
It said the system showed that at 02:26 local time on Thursday (00:26 GMT) smoke was detected in the Airbus A320 toilet.
A minute later - at 00:27 GMT - there was an avionics smoke alert.
The last ACARS message was at 00:29 GMT, the air industry website said, and the contact with the plane was lost four minutes later at 02;33 local time.
ACARS is used to routinely download flight data to the airline operating the aircraft.
Confirming the data, France's Bureau of Investigations and Analysis told AFP it was "far too soon to interpret and understand the cause of the accident as long as we have not found the wreckage or the flight data recorders".
Agency spokesman Sebastien Barthe told Associated Press the messages "generally mean the start of a fire" but added: "We are drawing no conclusions from this. Everything else is pure conjecture."
Philip Baum, the editor of Aviation Security International Magazine, told the BBC that technical failure could not be ruled out.
"There was smoke reported in the aircraft lavatory, then smoke in the avionics bay, and over a period of three minutes the aircraft's systems shut down, so you know, that's starting to indicate that it probably wasn't a hijack, it probably wasn't a struggle in the cockpit, it's more likely a fire on board."
Analysis: Richard Westcott, BBC transport correspondent
This data could be the biggest clue yet as to what happened. It suggests there was a fire at the front of the aircraft, on the right-hand side.
The sequence begins with a warning of an overheating window in the cockpit. Smoke is then detected in the lavatory (we assume it's the one behind the cockpit) and in a bay right underneath the cockpit, which is full of electronic equipment.
Finally, another window becomes too hot, before all the systems begin collapsing. All of this takes place over a few minutes, then the aircraft drops off the radar.
Some pilots have suggested that the 90 degree left turn the plane then made is a known manoeuvre to get out of the way in an emergency, when an aircraft needs to drop height suddenly.
The 360 degree turn after that, they say, could be the crew managing a crisis.
So it seems that the aircraft caught fire and that the fire spread very quickly. But whether that fire was deliberate or mechanical, we still can't say.
Security consultant Sally Leivesley said the timing on the data suggested an "extremely rapidly developing flame front from a fire that has overwhelmed the avionics very, very quickly".
She cited the case of "underpants bomber" Umar Farouk Abdulmutallab, who tried to set off an explosive device hidden in his underwear on a Detroit-bound flight in 2009.
Although the attempt failed, a fire from the device's chemicals still spread "right up the side of the plane".
The focus of the investigation
Greece says radar shows the Airbus A320 making two sharp turns and dropping more than 25,000ft (7,620m) before plunging into the sea.
The search is now focused on finding the plane's flight recorders, in waters between 2,500 and 3,000 metres deep.
But aviation expert Geoffrey Thomas warned there might be a "very long search", citing the case of Air France 447.
That plane crashed in the Atlantic in 2009 at a similar depth to the area in which the EgyptAir jet is thought to have gone done down. It took two years to find the main body of wreckage and the black boxes.
What do we know about what happened?
Three French air investigators, along with a technical adviser from Airbus, have joined the Egyptian inquiry.
The BBC has learned the plane that disappeared was forced to make an emergency landing in 2013 after the pilot noticed the engine overheating, but an official report said the defect had been repaired.
In October, an Airbus A321 operated by Russia's Metrojet blew up over Egypt's Sinai Peninsula, with all 224 people on board killed.
Sinai Province, a local affiliate of the Islamic State jihadist group, said it had smuggled a bomb on board.
Who were the victims?
The names of some of those who were on board have emerged, but most have not been identified publicly.
Those on board included:
- Richard Osman, a 40-year-old geologist and father-of-two from South Wales;
- Canadian national Marwa Hamdy, a mother-of-three and an executive with IBM originally from Saskatchewan, but who had relocated to Cairo;
- Pascal Hess, a photographer from Normandy, France, who had lost his passport last week - only for it to be found in the street, allowing him to catch the flight;
- An unnamed couple in their 40s from Angers in north-west France, as well as their two children;
- Ahmed Helal, the Egyptian-born manager of a Procter and Gamble plant in Amiens, northern France
If anyone is concerned about relatives or friends following the disappearance of the flight, they can call this free number provided by EgyptAir: +202 259 89320