Smoke was detected in multiple places in EgyptAir Flight 804 before it crashed in the Mediterranean, the French air accident investigation agency said.
Spokesman Sebastien Barthe told The Associated Press that the plane's automatic detection system sent messages indicating smoke a few minutes before it disappeared from radar.
The messages, he said, "generally mean the start of a fire".
But he added: "We are drawing no conclusions from this. Everything else is pure conjecture."
Search teams found floating human remains, luggage and seats from the Airbus A320 on Friday, but face a potentially more complex task in locating bigger pieces of wreckage and the black boxes vital to determining why the plane went down.
Looking for clues to whether terrorists brought down EgyptAir Flight 804 and the 66 people aboard, investigators pored over the passenger list and questioned ground crew members at Charles de Gaulle Airport in Paris, where the plane took off for Cairo.
The Airbus had been cruising normally in clear skies on the night-time flight 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.
Meanwhile search crews were continuing to scour for further wreckage of the plane – including for the black boxes, which could provide vital clues to why the jetliner crashed killing all 66 on board.
Planes and vessels from Egypt and five other countries are searching a wide area of the Mediterranean, a day after the Egyptian army found debris from the Airbus 320 in the sea 180 miles (290 kilometers) north of Alexandria.
No hard evidence has emerged as to why the plane dropped off radar, swerved wildly and plummeted early on Thursday morning.
Investigators are considering the possibility of a terror attack, though no militant group claims to have brought down the plane.
In Egypt, home to 30 of the victims, grieving families and friends wondered if their loved ones would ever be recovered. Many gathered in mosques for Salat al-Ghaib, or "prayers for the absent", held for the dead whose bodies have not been found.
Egyptian authorities believe terrorism is a more likely explanation than equipment failure and some aviation experts have said the erratic flight suggests a bomb blast or a struggle in the cockpit. But so far no hard evidence has emerged.
Amid fears that a security lapse in Paris may have led to the tragedy, France's junior minister for transport, Alain Vidalies, defended security at De Gaulle Airport, saying staff badges are revoked if there is the slightest doubt.
Foreign Minister Jean-Marc Ayrault asserted on French television that there was "absolutely no indication" of what caused the crash.