Egyptian President Abdel Fattah al-Sisi said it was to early to tell what caused the Airbus A320 to plunge into the sea with 66 people on board, after it sent out automated messages signalling smoke on board and a pilot control system fault.
Some wreckage from the plane that had been en route to Cairo from Paris has been found, but none of the passengers' bodies, an Egyptian civil aviation ministry spokesman said on Sunday.
Sisi said a submarine that can operate 3,000 metres (9,800) below sea level had been enlisted from the oil ministry, and urged against speculation on why the plane went down.
"All the theories are possible," he said in a televised address.
"There is no particular theory we can affirm right now," he added.
What brought down the plane remains the main question.
Egypt's aviation minister has said terrorism is more likely than technical failure without offering any proof, and although most aviation disasters were not caused by terrorism.
French Foreign Minister Jean-Marc Ayrault said "all theories are being examined and none is favoured".
Relatives and friends of the cabin crew and passengers, who were on board the EgyptAir plane that crashed in the Mediterranean, attend a mourning ceremony on Sunday. Photo: AFP
The disaster comes just seven months after the bombing of a Russian airliner over Egypt's Sinai peninsula in October that killed all 224 people on board.
The Islamic State group has claimed responsibility for that attack within hours, but there has been no claim linked to the EgyptAir crash.
A rare audio message on Saturday by the jihadist group's spokesman Mohamed al-Adnani made no mention of the plane, although it called for attacks on the US and European countries, especially civilians, during the Muslim fasting month of Ramadan which begins in June.
The message may have been recorded before the disaster on Thursday.
A French patrol boat carrying equipment capable of tracing the black boxes was expected in the search area between the Greek islands and the Egyptian coast on Monday afternoon.
But experts have warned that the equipment could be useless if the black boxes -- which can emit signals up to five weeks -- have sunk to a depth of more than 2,000 metres (6,500 feet).11
French and Egyptian aviation officials have said it is too soon determine what brought down the plane.
It is "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", a spokesman for France's Bureau of Investigations and Analysis (BEA) said.
An Egyptian aviation ministry statement said it was too early to draw any conclusions based on a "single source of information such as the ACARS messages which are signals and indicators that may have different causes".
ACARS -- Aircraft Communications Addressing and Reporting System -- transmits short messages between aircraft and ground stations.
The ACARS messages read "smoke lavatory smoke" then "avionics smoke" -- referring to the plane's electronic systems -- then a "fault" with the FCU, the pilots' flight control unit in the cockpit and another control unit, a official with BEA told AFP.
The plane disappeared between the Greek island of Karpathos northeast of Crete and the Egyptian coast in the early hours of Thursday.
It had turned sharply twice before plunging 6,700 metres (22,000 feet) and vanishing from radar screens, said Greek Defence Minister Panos Kammenos.
The passengers were 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 and three security personnel were also on board.
EgyptAir Holding Company chairman Safwat Moslem told AFP the priority was finding the passengers' remains and the flight recorders, which will stop emitting a signal in a month when the batteries run out.
"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.
On Saturday, a funeral service was held at a church in Cairo for 26-year-old EgyptAir hostess Yara Hany Farag.
In a hall inside the church compound decorated with flowers, a picture of a smiling Yara was placed on a cross covered entirely with white flowers. The young woman was engaged and due to be married in the coming months.