An EgyptAir jet carrying 66 passengers and crew from Paris to Cairo disappeared from radar over the Mediterranean south of Greece on Thursday, with Athens saying the plane swerved in mid-air before plunging from cruising height and vanishing.
Egyptian Prime Minister Sherif Ismail said it was too early to rule out any explanation, including an attack like the one blamed for bringing down a Russian airliner over Egypt's Sinai Peninsula last year.
The country's aviation minister said a terrorist attack was more likely than a technical failure.
Greece deployed aircraft and a frigate to search for the missing Airbus and officials said they had found pieces of plastic and two lifevests that appeared to have come from an aircraft in the sea 230 miles (370 km) south of Crete.
Egypt said it would lead the investigation and that France would participate. Other countries also offered to help, including Britain and the United States.
In Washington, President Barack Obama received a briefing on the disappearance from his adviser for homeland security and counter-terrorism, the White House said.
Greek Defence Minister Panos Kammenos said the Airbus had first swerved 90 degrees to the left, then spun through 360 degrees to the right. After plunging from 37,000 feet to 15,000, it vanished from Greek radar screens.
According to Greece's civil aviation chief, calls from Greek air traffic controllers to flight MS804 went unanswered just before it left Greek airspace, and it disappeared from radar screens soon afterwards.
There was no official indication of a possible cause, whether technical failure or sabotage by ultra-hardline Islamists who have targeted airports, airliners and tourist sites in Europe, Egypt, Tunisia and other Middle Eastern countries over the past few years.
The aircraft was carrying 56 passengers - with one child and two infants among them - and 10 crew, EgyptAir said. They included 30 Egyptian and 15 French nationals, along with citizens of 10 other countries.
Asked if he could rule terrorist involvement, Prime Minister Ismail told reporters: "We cannot exclude anything at this time or confirm anything. All the search operations must be concluded so we can know the cause."
French President Francois Hollande also said the cause was unknown. "No hypothesis can be ruled out, nor can any be favoured over another."
With its archaeological sites and Red Sea resorts, Egypt is a traditional destination for Western tourists. But the industry has been badly hit by the downing of a Russian Metrojet flight last October, in which all 224 people on board were killed, as well as by an Islamist insurgency and a string of bomb attacks.
Greek air traffic controllers spoke to the pilot as the jet flew over the island of Kea, in what was thought to be the last broadcast from the aircraft, and no problems were reported.
But just ahead of the handover to Egyptian controllers, calls to the plane went unanswered.
"About seven miles before the aircraft entered the Cairo airspace, Greek controllers tried to contact the pilot but he was not responding," said Kostas Litzerakis, head of Greece's civil aviation department. Shortly after exiting Greek airspace, it disappeared from radars, he said.
Greek authorities were searching the sea south of the island of Karpathos, Defence Minister Kammenos told a news conference.
"At 3:39 a.m. (0039 GMT), the course of the aircraft was south and southeast of Kassos and Karpathos (islands)," he said. "Immediately after, it entered Cairo FIR (flight information region) and made swerves and a descent I describe: 90 degrees left and then 360 degrees to the right."
The Airbus plunged from a height of 37,000 feet (11,280 metres) to 15,000 feet before vanishing from radar, he added.
Egyptian Civil Aviation minister Sherif Fathi said authorities had tried to resume contact but without success.
"NO ONE KNOWS ANYTHING"
At Cairo airport, authorities ushered families of the passengers and crew into a closed-off waiting area.
Two women and a man, who said they were related to a crew member, were seen leaving the VIP hall where families were being kept. Asked for details, the man said: "We don’t know anything, they don’t know anything. No one knows anything."
Ayman Nassar, from the family of one of the passengers, also walked out of the passenger hall with his daughter and wife in a distressed state. "They told us the plane had disappeared, and that they’re still searching for it and not to believe any rumours," he said.
The mother of a flight attendant rushed out of the hall in tears. She said the last time her daughter called her was Wednesday night. "They haven’t told us anything," she said.
In Paris, a police source said investigators were now interviewing officers who were on duty at Roissy airport on Wednesday evening to find out whether they heard or saw anything suspicious. "We are in the early stage here," the source said.
Airbus said the missing A320 was delivered to EgyptAir in November 2003 and had operated about 48,000 flight hours.
The missing flight's pilot had clocked up 6,275 hours of flying experience, including 2,101 hours on the A320, while the first officer had 2,766 hours, EgyptAir said.
At one point, EgyptAir said the plane had sent an emergency signal at 04:26 a.m., two hours after it disappeared from radar screens. However, Fathi said later that further checks found that no SOS was received.
FRANCE AND EGYPT TO COOPERATE
The weather was clear at the time the plane disappeared, according to Eurocontrol, the European air traffic network.
Under U.N. aviation rules, if the aircraft is found to have crashed in international or Egyptian waters, Egypt will automatically lead an investigation into the accident, assisted by countries including France, where the jet was assembled, and the United States, where engine maker Pratt & Whitney is based.
Russia and Western governments have said the Metrojet plane that crashed on Oct. 31 was probably brought down by a bomb, and the Islamic State militant group said it had smuggled an explosive device on board.
That crash called into question Egypt's campaign to contain Islamist violence. Militants have stepped up attacks on Egyptian soldiers and police since Sisi, then serving as army chief, toppled elected president Mohamed Mursi, an Islamist, in 2013 after mass protests against his rule.
In March, an EgyptAir plane flying from Alexandria to Cairo was hijacked and forced to land in Cyprus by a man with what authorities said was a fake suicide belt. He was arrested after giving himself up.
EgyptAir has a fleet of 57 Airbus and Boeing jets, including 15 of the Airbus A320 family of aircraft, according to airfleets.com.