CAIRO — The EgyptAir red-eye from Paris to Cairo, an Airbus A320 jetliner less than half full, had just entered Egyptian airspace early Thursday on the final part of its journey.

Suddenly the twin-engine jetliner jerked hard to the left, then hard to the right, circled and plunged 28,000 feet, disappearing from the radar screens of Greek and Egyptian air traffic controllers.

That began a day of emergency rescuers scrambling, officials issuing conflicting information and experts speculating about the fate of EgyptAir Flight 804, which carried at least 66 people from roughly a dozen nations and was presumed to have crashed into the Mediterranean Sea.

EgyptAir initially said wreckage of the plane had been found with the help of searchers from Greece, but a senior official of the airline speaking on CNN retracted that assertion hours later. Egyptian officials suggested that terrorism was a more likely cause for the disappearance than mechanical failure, but others cautioned that it was premature to make that judgment.