Experts said the “wide dispersion of wreckage discovered suggests that the Airbus exploded at high altitude”.
Terrorism has not been ruled out, but they said the most likely scenario was that the break-up was caused by massive depressurisation inside the plane.
If depressurisation had occurred at high altitude, passengers would have almost certainly fallen unconscious instantly and may have been unaware of their fate.
Professor Philippe Juvin, of Beaujon hospital, west of Paris, said: “It would have been as quick as the moment when one falls asleep.”
Depressurisation can be caused by failure of the pressure control system, reduced cabin air inflow, or structural failure — such as an open door, a cracked window, or a hole caused by a bomb.
The structure can also disintegrate if the G-forces during a dive are more than the plane can cope with.
Investigators will examine a bomb threat made against a flight from Buenos Aires to Paris just days before Flight 447 disappeared.
One unnamed Air France pilot suggested that a bomb could be the cause of the crash.
He said: “One can very well imagine that a bomb caused the aircraft's depressurisation and that the plane took time to break up. It could just as well have been a big bomb that blew up the entire plane, which would explain why the aircraft didn't have time to send an alert signal.”
"If there was an explosion on board, the wreckage would have been spread over a very wide area, as it was.
"So in my opinion there is no other option than the highly likely theory that a bomb went off on the plane - perhaps even a large bomb that destroyed it in mid-air leaving no chance to send a message."
He added: "I have flown these jets for more than ten years and the chances of an electrical fault seem unfeasible.
"There are five electricity supplies on the plane. And to cause it to break down completely they would all have to fail. Even then, a type of wind turbine takes over to generate power.
"We cannot know if it crashed after being struck by lightning. But we know this is very rare."
Crash investigators said they were “not optimistic” about retrieving the plane's black boxes, despite confirmation that debris spotted 400 miles off Brazil's coast came from the missing plane.
Paul-Louis Arslanian, of the French civil aviation ministry, said it would be very difficult to recover the flight data recorders because of the depth of the ocean — up to 10,000ft — and its rugged floor.
“The investigation will not be easy — but we are not giving up,” he said.
A French mini-submarine will arrive at the area next week.