EgyptAir Flight 804 vanished from radar on its way from Paris to Cairo with 66 people aboard, the airline said Thursday.
The plane was flying at 37,000 feet when it lost contact overnight above the Mediterranean Sea, the airline tweeted. French President Francois Hollande said he had been told the flight crashed.
It's too early in the investigation to say for sure what caused the disaster, and at this point experts say anything from mechanical failure to the terrorism is a possibility.
"No hypothesis has been ruled out," Hollande said.
For now, finding the airplane and any possible survivors is the priority.
Somber relatives gathered in Cairo and Paris airports, seeking word on their loved ones. They were ferried to special centers at both airports, where translators and psychiatric support awaited. In Cairo's airport, dozens of relatives paced anxiously in a building set aside for families. Others shouted at photographers taking pictures of them while some berated officials over the perceived lack of information.
--The airplane "swerved and then plunged" before descending into the Mediterranean, Greece's defense minister told reporters.
-- Greek controllers tried to reach EgyptAir Flight 804 about 10 miles before it left the country's airspace and for about 90 seconds after and received no response, the head of Greek Civil Aviation told Greek broadcaster ANT1 TV.
-- Prosecutors in Paris have opened an investigation into the disappearance of the plane, their office said in a statement. "No hypothesis is privileged or pushed aside for the moment," the statement said.
The flight seemed to be proceeding normally until it approached Egyptian airspace. Greek controllers talked to the pilot when the plane was near the Greek island of Kea at 37,000 feet at an air speed of 519 mph. Everything seemed fine at that point.
At 3:27 a.m. local time, shortly before the aircraft was scheduled to exit Greek airspace, controllers tried to reach the pilots to transfer control to Cairo authorities. Despite repeated attempts, they received no response, the Hellenic Civil Aviation Authority said. The plane passed into Egyptian airspace two minutes later. Forty seconds later, radar contact was lost, the authority said.
Weather conditions were clear at the time, CNN meteorologist Pedram Javaheri said.
More about the flight
The flight left Charles de Gaulle Airport at 11:09 p.m. Wednesday local time and was supposed to land in Cairo at 3:15 a.m. Thursday. Both the departure and arrival cities are in the same time zone.
Passengers and crew were from France, Egypt, Britain, Belgium, Iraq, Kuwait, Saudi Arabia, Sudan, Chad, Portugal, Algeria and Canada.
A majority were French and Egyptian -- 15 and 30 people respectively, EgyptAir said in a statement. Among those aboard: three children -- two of them infants, said Capt. Ahmed Adel, a vice chairman at EgyptAir.
The Airbus A320 had routine maintenance checks Wednesday in Cairo, before it left for Paris, an airline official said. Earlier Wednesday, the jet was also in Eritrea and Tunisia, data from flight tracking websites show.
There was no special cargo on the flight and no notification of any dangerous goods aboard, Adel said.
The plane has been part of EgyptAir's fleet since November 3, 2003, according to Adel. It had about 48,000 flight hours. The plane's captain had about 6,000 flying hours, he said.
Egyptian and Greek military vessels and aircraft are looking for the aircraft and any possible survivors. Greece is looking about 130 nautical miles southeast of Karpathos island.
A distress signal was detected at 4:26 a.m. -- about 2 hours after the jet vanished -- in the general vicinity where it disappeared, Adel said.
He said the distress signal could have come from another vessel in the Mediterranean. Egyptian armed forces stressed that they had not received a distress call.
If there are any survivors, there's still a window to save them.
"The water temperatures in the eastern Mediterranean near Egypt are in the low 20s Celsius [mid to low 70s Fahrenheit]," Javaheri said.
"Survival times in such waters range from 2-7 hours for the elderly or individuals in poor health, while they range anywhere from 2 to 40 hours for healthier individuals."
A storm system could affect conditions in the region as early as Friday afternoon," Javaheri said.
Analysts weigh in
CNN aviation correspondent Richard Quest: "Planes just do not fall out of the sky for no reason, particularly at 37,000 feet," he said, noting the plane vanished while cruising -- the safest part of the journey.
David Soucie, a CNN aviation safety analyst: The first priority is to find survivors. "Find the plane, find the people, see if there are folks that could be rescued," he said. "Safety people are looking at safety issues, maintenance people looking at maintenance issues, security people looking at security issues."
CNN aviation analyst Les Abend: He says there are three possibilities: Explosion, something nefarious or a stall situation. "We're in the very early stages of the investigation. Any good accident investigator will tell you, just put on the brakes a little bit and let this thing unfold. The 360-degree turn, that seems very abrupt. It's not something I would do in any major emergency unless I was losing control of the aircraft."
Egypt's aviation incidents
Egypt is no stranger to aviation disasters.
In March, an "unstable" man diverted an EgyptAir flight from Alexandria to Cyprus. The suspected hijacker later released all hostages and surrendered.
Last year, a Russian plane exploded midair over the Sinai Peninsula, killing all 224 people aboard. Egyptian officials initially downplayed Islamic militants' claim that they brought down the jet, saying technical failure caused the crash.
And in 1999, an EgyptAir passenger jet made a rapid descent, plunging almost 14,000 feet in 36 seconds.
The Boeing 767, en route to Cairo from New York City, crashed into the Atlantic Ocean off the Massachusetts coast in October of that year.
Though its debris was later found, speculation remains on the cause of the crash that killed all 217 people on board.