Three French aviation accident investigators and an Airbus technical expert arrived in Cairo on Friday to join an Egypt-led probe into the disappearance of an Egyptair jet with 66 people on board the day before.
The Airbus A320, travelling from Paris to Cairo, disappeared from radar around 2:30 am (0030 GMT) Thursday, prompting a massive international search in the eastern Mediterranean Sea.
"The participation of the French investigators in the probe is because 15 of the 56 passengers who were on board were French citizens and France is the headquarters of the plane manufacturer," an official at Cairo airport said on condition of anonymity.
Egyptian President Abdel-Fattah al-Sissi instructed a state-run inquiry commission to start investigating the accident, a presidential office said late Thursday.
Ships and planes from France, Greece and Egypt have been scouring the area where the Egyptair flight MS804 is thought to have come down, about 280 kilometres from the Egyptian mainland.
Search operations could be complicated by heavy rainfall and strong winds moving into the eastern Mediterranean on Friday, Greece's weather service said.
Greek Prime Minister Alexis Tsipras vowed to cooperate closely with Egypt to find the plane. He offered condolences and support to al-Sissi during a phone call, Tsipras' office said in a statement.
Multiple reports of debris in the hours after the apparent crash were found not to have been from the passenger jet, which was carrying 56 passengers, seven crew members and three security officers.
One passenger identified as British was confirmed Friday to be a dual Australian-British national, the Australian government said.
French Foreign Minister Jean-Marc Ayrault told the France2 broadcaster that there were "absolutely no clues" to indicate the reason for the plane's disappearance, adding that he would meet with victims' families this weekend in Paris.
The pilots did not make contact when they left Greek airspace as is customary, Egypt's aviation authority said. The jet was flying at a height of 37,000 feet (11,300 metres) when it disappeared about 45 minutes before its expected landing in Cairo.
The likelihood that it was the victim of a terrorist attack was "far higher than the likelihood that the plane developed a technical failure," Egyptian Civil Aviation Minister Sherif Fathy said.
The presumed crash comes in the wake of two other incidents involving Egyptian aviation.
In March, an Egyptian man used a fake explosive belt made of mobile phone covers and cables to force an Egyptair flight to land in Cyprus.
In October 2015, a Russian passenger jet broke up in mid-air shortly after take-off from the resort of Sharm el-Sheikh in Egypt's Sinai Peninsula, killing all 224 people on board.
Russian authorities said that the incident was caused by a bomb. The Islamic State extremist group, which operates in Sinai, claimed responsibility and published a photograph of a soft drink can which it said had been filled with explosives and smuggled onto the flight.