According to the preliminary report prepared by the agency, a study of 660 pieces of debris showed that it had shattered only when it slammed belly-first into the sea.
Chief Investigator Alain Bouillard, addressing news conference, said: "The plane went straight down, almost vertically towards the surface of the water, very fast".
The 228 passengers on board the Airbus A300 were not prepared for an emergency as none of them were wearing life-jackets, Bouillard said, adding that there was no way of knowing if they were conscious when the aircraft hit the water.
When pressed by media persons on the reason why the aircraft crashed, Bouillard said it will be difficult if not impossible to gain a clear picture of what happened during the critical last minutes high over the Atlantic unless the aircraft's flight recorders were found and analysed.
"They normally give a signal for 30 days. We will keep listening another 10 days," he said.
However, Bouillard did indicate at apparent failure of pitot tubes (small devices affixed to the plane's skin to gauge airspeed) as one amongst several other factors like high winds, thunder, lightning and possible equipment failures or human error.
According to the Financial Times, Air France had come under fire for not having replaced earlier the pitot tubes on all its A330s and A340s, as its own service bulletin had recommended in 2007. The airline ordered their replacement only after flight AF447 crashed.
Image: Rescue team workers salvage a debris of the Air France aircraft.
Photograph: Reuters