A fire on board would likely have generated multiple warning signals, while a sudden explosion may not have generated any—though officials stress that no scenario, including explosion, is being ruled out.
An EgyptAir jet sent a series of signals indicating that smoke had been detected on board before it crashed into the Mediterranean on Thursday, France's air accident investigation agency said on Saturday.
A spokesman for the BEA agency said the signals did not indicate what caused the smoke or fire on board the plane, which plunged into the sea with 66 people on board as it was heading from Paris to Cairo.
But they offered the first clues as to what unfolded in the moments before the crash.
One aviation source said that a fire on board would likely have generated multiple warning signals, while a sudden explosion may not have generated any—though officials stress that no scenario, including explosion, is being ruled out.
Egypt said its navy had found human remains, wreckage and the personal belongings of passengers floating in the Mediterranean about 290 km (180 miles) north of Alexandria.
The army published pictures on Saturday on its official Facebook page of the recovered items, which included blue debris with EgyptAir markings, seat fabric with designs in the airline's colours, and a yellow lifejacket.
Analysis of the debris and recovery of the black boxes are likely to be key to determining the cause of the crash—the third blow since October to Egypt's travel industry, still reeling from political unrest following the 2011 uprising that ousted Hosni Mubarak.
A suspected Islamic State bombing brought down a Russian airliner after it took off from Sharm el-Sheikh airport in late October, killing all 224 people on board, and an EgyptAir plane was hijacked in March by a man wearing a fake suicide belt.
The October crash devastated Egyptian tourism, a main source of foreign exchange for a country of 80 million people, and another similar incident would crush hopes of it recovering.
Hunt for black boxes
The signals from the plane "do not allow in any way to say what may have caused smoke or fire on board the aircraft," said a spokesman for the French BEA agency, which is assisting an official Egyptian investigation.
He added that the priority now was to find the two flight recorders, known as black boxes, containing cockpit voice recordings and data readings, from the Airbus A320 which vanished from radar early on Thursday.
The plane is believed to have come down in a deep part of the Mediterranean, in 2,000 to 3,000 metres of water, according to a person familiar with Western naval estimates.
That would place the black box locator beacons, which last for 30 days, on the edge of their detectable range from the surface using acoustic equipment typically used during the first stages of a search, according to a report into the 2009 crash of an Air France jet in the Atlantic.
The flight data transmitted before the crash was sent through an automatic system called the Aircraft Communications Addressing and Reporting System (ACARS), which routinely downloads maintenance and fault data to the airline operating the aircraft.
Aviation website Aviation Herald published a burst of seven messages broadcast over the space of three minutes. These included alarms about smoke in the lavatory as well as the aircraft's avionics area, which sits under the cockpit.
While suggesting a possible fire, the relatively short sequence of data gives no insight into pilot efforts to control the aircraft, nor does it show whether it fell in one piece or disintegrated in mid-air, two aviation safety experts said.
"The question now is whether the fire that caused the smoke was the result of an electrical fault —for example a short-circuit caused by damaged wiring—or whether some form of explosive or incendiary device was used—for example by a terrorist—to generate a fire or other damage," aviation safety expert David Learmont said.
The ACARS data suggested the fire had spread fast and "that might explain the fact that there was no distress call," Learmont wrote in a blog.
The aircraft was carrying 56 passengers, including a chid and two infants, and 10 crew, EgyptAir said on Thursday. They included 30 Egyptian and 15 French nationals, along with citizens of 10 other countries.
"At this very moment all scenarios are being examined and none is being given greater emphasis," French Foreign Affairs Minister Jean-Marc Ayrault told reporters after meeting relatives of the victims of the EgyptAir crash.
“Searching for the aircraft is the priority, finding the 'black boxes' will help answer the legitimate questions raised (by the families)," Ayrault said.
France sent a plane and a navy ship to assist with the searches for the jet and three representatives of the BEA, together with an expert from Airbus, were in Cairo to help with the investigation.
The naval search is centred on an area north of Alexandria, just south of where the signal from the plane was lost early on Thursday.
EgyptAir Chairman Safwat Moslem told state television that the radius of the search zone was 40 miles, giving an area of 5,000 square miles, but said it may be expanded.
A European satellite spotted a 2 km-long oil slick in the Mediterranean, about 40 km southeast of the aircraft's last known position, the European Space Agency said.