Officials say the Air France jet that crashed one month ago did not break up in the air but rather plunged vertically at a very high speed into the Atlantic Ocean.

Alain Bouillard, with the French accident investigation agency BEA, said the plane "seems to have hit the surface of the water on its flight trajectory with a strong vertical acceleration."

He added that investigators have found "neither traces of fire nor traces of explosives."

Bouillard also said life vests found among the wreckage were not inflated. He said that suggests that the passengers were not prepared for a crash landing in the water.

"Today we are very far from establishing the causes of the accident," Bouillard told reporters at a press conference in Paris Thursday.

The Airbus A330-200 plane was on its way from Rio de Janeiro, Brazil to Paris when it vanished over the ocean with 228 people on board.

In total, Bouillard said 51 bodies have been recovered, along with 600 elements of the plane.

However, search teams have been unable to locate the plane's black boxes, which would help investigators better understand what happened.

Bouillard said the search for the boxes will continue for another 10 days, even though the signals they emit are likely to have already faded out.

Bouillard did say speed sensors, called Pitot tubes, were a factor in the crash but were not the cause.

One of the automatic messages sent out by the plane indicates it was receiving incorrect speed information from its external monitoring instruments.

Experts say those external instruments may have iced over and the wrong information could have destabilized the plane's control systems.

All of the passengers on the plane were killed in the crash.

With files from The Associated Press