EgyptAir flight MS804 sent automated messages signalling smoke onboard before plunging into the Mediterranean, the French aviation safety agency said on Saturday, as search teams hunted for more wreckage.
The Airbus A320 carrying 66 people had been flying from Paris to Cairo early Thursday when it plummeted and turned full circle before vanishing off radar, with some debris later found in the Mediterranean Sea.
While the automated messages may offer clues, more wreckage including the black boxes will need to be found before investigators can piece together what happened.
"There were ACAR messages emitted by the plane indicating that there was smoke in the cabin shortly before data transmission broke off," a spokesman for France's Bureau of Investigations and Analysis told AFP.
ACAR, which stands for Aircraft Communications Addressing and Reporting System, is a digital system that transmits short messages between aircraft and ground stations.
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," he added.
Search teams were scouring the sea on Saturday for more parts of the plane and the black boxes for clues on why it came down.
An EgyptAir official said the search was focused on finding the bodies of the passengers and the flight recorders, which will stop emitting a signal in a month when the batteries run out.
While Egypt's aviation minister has pointed to terrorism as more likely than technical failure, French Foreign Minister Jean-Marc Ayrault said on Friday there was "absolutely no indication" of why the plane crashed.
"We're looking at all possibilities," he said.
The disaster comes just seven months after the bombing of a Russian passenger jet by the Islamic State jihadist group over Egypt's Sinai peninsula in October that killed all 224 people on board.
'Families want the bodies'
Families of the passengers on the EgyptAir flight gathered at a hotel near Cairo airport after meeting airline officials as they struggled to come to terms with the catastrophe.
"They haven't died yet. No one knows. We're asking for God's mercy," said a woman in her 50s whose daughter had been on board.
On Saturday, EgyptAir Holding Company chairman Safwat Moslem told AFP that the priority was finding the passengers' remains and the black boxes.
"The families want the bodies. That is what concerns us. The army is working on this. This is what we are focusing on," he said.
French investigators met their Egyptian counterparts in Cairo, while a French patrol boat carrying equipment capable of tracing the plane's black boxes was expected on Sunday or Monday.
The plane disappeared between the Greek island of Karpathos and the Egyptian coast in the early hours of Thursday, without its crew sending a distress signal.
It had turned sharply twice before plunging 22,000 feet (6,700 metres) and vanishing from radar screens, said Greek Defence Minister Panos Kammenos.
Boy, babies on board 
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," he said.
"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. Now whether that was a technical fire, a short circuit, or whether it was because a bomb went off on board, we don't know."
Greek civil aviation chief Constantinos Litzerakos said the pilot had mentioned no problem in his last communication.
"The flight controllers contacted the pilot at a height of 37,000 feet... he did not mention a problem," he said.
Personal belongings and parts of the Airbus A320 were spotted by teams searching the sea off Egypt's northern coast about 290 kilometres from the city of Alexandria, the military said.
Kammenos said the teams, which include multinational aircraft and ships, had found "a body part, two seats and one or more items of luggage".
The passengers included 30 Egyptians, 15 French citizens, two Iraqis, two Canadians, and citizens from Algeria, Belgium, Britain, Chad, Portugal, Saudi Arabia and Sudan. They included a boy and two babies.
Seven crew members and three security personnel were also on board.
The European Space Agency said one of its satellites had on Thursday spotted an oil slick about 40 kilometres southeast of the plane's last known location.
In October, foreign governments issued travel warnings for Egypt and demanded a review of security at its airports after the Islamic State group said it downed the Russian airliner over Sinai with a bomb concealed in a soda can that had been smuggled on the plane.
IS has been waging a deadly insurgency against Egyptian security forces and has claimed attacks in both France and Egypt.