Smoke was detected in multiple places on EgyptAir flight 804 moments before it plummeted into the Mediterranean.
But the cause of the crash that killed all 66 on board remains unclear, the French air accident investigation agency says.
Agency spokesman Sebastien Barthe told The Associated Press that the plane's automatic detection system sent messages indicating smoke a few minutes before the plane disappeared from radar while flying over the east Mediterranean early on Thursday morning.
The messages, he explained, "generally mean the start of a fire", but he added: "We are drawing no conclusions from this. Everything else is pure conjecture."
The industry publication Aviation Herald also reported that sensors detected smoke in the plane's lavatory, suggesting a fire onboard.
Looking for clues to whether terrorists may have brought down the Airbus A320, investigators have been poring over the passenger list and questioned ground crew members at Charles de Gaulle Airport in Paris, from which the plane took off.
The Airbus A320 had been cruising normally in clear skies on a night flight to Cairo early Thursday when it suddenly lurched left, then right, spun all the way around and plummeted 38,000 feet into the sea, never issuing a distress signal.
Search crews, meanwhile, are scouring for further wreckage of the aircraft, including its black boxes, which could provide vital clues to why the jetliner crashed.
Planes and vessels from Egypt and five other countries continued searching a wide area of the eastern Mediterranean on Saturday, a day after the Egyptian army found debris from the passenger jet in the sea 290 kilometres north of the Egyptian port city of Alexandria.
