PARIS  The Air France Airbus A330 that crashed into the Atlantic on June 1, killing all 228 people aboard, did not break up in the air but rather hit the water intact, French investigators said Thursday.

But at a news conference at their headquarters at Le Bourget airport near Paris, officials from the French Office of Investigations and Analyses acknowledged that they still had no clear understanding of the reason for the crash of Air France Flight 447. The plane was flying through an area of strong thunderstorms when it went down 600 miles off northern Brazil en route to Paris.

Analysis of autopsies and debris patterns in the weeks after the crash had seemed to bolster speculation that the plane had broken up in flight. But the investigators said their examination of floating debris indicated that the plane plummeted on its belly onto the ocean surface, facing in the direction of its intended route.

Alain Bouillard, who is leading the French investigation, said that “visual examination of the debris shows that the plane hit with the bottom of its fuselage with very strong vertical acceleration.” Among the evidence was that shelves in the galley had compressed to the bottom, he said.