An EgyptAir plane travelling from Paris to Cairo with 66 people on board swerved wildly in flight and crashed in the Mediterranean Sea.
Here's everything we know so far:
What happened to the plane?
Greek defence minister Panos Kammenos said the plane spun and suddenly lost altitude just before vanishing from radar screens around 2.45am Cairo time on Thursday.
He said it made a 90-degree left turn, then a full 360-degree turn towards the right, plummeting from 38,000ft to 15,000ft and then disappearing at about 10,000ft.
The Egyptian military said it did not receive a distress call, and reports claimed the pilot did not send one. The absence of a distress call suggests that whatever sent the aircraft plummeting into the sea was sudden and brief.
What caused the crash?
The plane's erratic course raised a number of possibilities, including a catastrophic mechanical or structural failure, a bombing or a struggle over the controls with a hijacker in the cockpit.
Egyptian security officials said they are running background checks on the passengers to see if any had links to extremists.
Egyptian Civil Aviation Minister Sherif Fathi cautioned the disaster is still under investigation but said the possibility it was a terror attack "is higher than the possibility of having a technical failure".
And Alexander Bortnikov, chief of Russia's top domestic security agency, went further, saying: "In all likelihood it was a terror attack."
If it was terrorism, it was the second deadly attack involving Egypt's aviation industry in seven months.
Last October, a Russian passenger plane that took off from an Egyptian Red Sea resort crashed in the Sinai, killing all 224 people aboard. Russia said it was brought down by a bomb, and a local branch of the Islamic State claimed responsibility.
Has anything been found?
Several hours later, Egypt's Civil Aviation Ministry said life jackets, plastic items and other floating objects had been found in the area where the plane went down.
But a senior Greek air safety official later said the debris found so far did not come from an aircraft.
Athanassios Binis, head of Greece's Air Accident Investigation and Aviation Safety Board, told ERT TV that "an assessment of the finds showed that they do not belong to an aircraft", and that had been confirmed by Egyptian authorities.
Egyptian military aircraft and ships are continuing to searched for debris and victims, joined by Greek, French and British authorities.
Who was on board?
Those on board, according to EgyptAir and various governments, included 15 French passengers, 30 Egyptians, two Iraqis, one Briton, one Kuwaiti, one Saudi, one Sudanese, one Chadian, one Portuguese, one Belgian, one Algerian and two Canadians. The passengers included two babies.
The Briton - Richard Osman, from Wales - was a kind and loving father of two. His younger brother Alastair Osman described him as a workaholic and a very admirable person who "never deviated from the straight path".
What does this disaster mean for security?
The disaster also raises questions about security at Charles de Gaulle Airport in Paris, at a time when western Europe has been on high alert over the deadly Islamic extremist attacks in Paris and at the Brussels airport and subway over the past six months.
French foreign minister Jean-Marc Ayrault said airport security had been tightened considerably before the disaster, in particular because of the upcoming European football championship which France is hosting.
And what does it mean for Egypt's tourism industry?
The crash also puts further pressure on Egypt's struggling tourism industry, which has been trying to recover after a string of other disasters.
In March a man wearing a fake suicide belt allegedly hijacked a domestic EgyptAir flight to Cairo, forcing it to divert to Cyprus. At least 88 people were killed and more than 200 injured in bomb attacks in Sharm in July 2005.
Bournemouth University's Dr Yeganeh Morakabati, who researches the perception of risk in relation to terrorism, said "it doesn't look good" for Egypt's tourist industry.
"They are in a very difficult place just now," she told the Press Association. "It's almost like there's a spell on Egypt, a series of bad events hitting the country."
Egypt launched a six-point recovery plan for its tourism sector in April after the number of overseas visitors fell by 40% in the first quarter of the year compared to the same period in 2015.