Search teams recovered 51 bodies from the crash area

French investigators trying to find out why an Air France plane crashed in the Atlantic say they believe it broke up on contact with water, not in the air.

They also found that the plane's speed sensors had been "a factor but not the cause" of the crash.

All 228 people on the plane were killed when it plunged into the ocean en route from Rio de Janeiro to Paris on 1 June.

Teams looking for the plane's flight data recorders will continue operations for another 10 days.

Alain Bouillard, of France's BEA accident investigation agency, said the crash had been an extremely difficult one to understand.

"Between the surface of the water and 35,000ft [10,700m], we don't know what happened," Mr Bouillard said. "In the absence of the flight recorders, it is extremely difficult to draw conclusions."

But he said an examination of the recovered wreckage led them to believe the plane probably hit the water "in the direction of flight and with a strong vertical acceleration".

BBC transport correspondent Tom Symonds said if the plane had broken up in the air, pieces of the fuselage would have been found twisted in a variety of directions.

Instead they showed signs of compression in one direction, resulting from the plane hitting the water on its belly.

Life jackets found in the wreckage had not been inflated, indicating that the passengers had little warning of a water landing.

Many factors

There has been speculation that the old-style speed sensors may have given the plane's pilots faulty information.

But Mr Bouillard said they had been "a factor but not the cause" of the crash.

In the wake of the crash, Air France accelerated an existing programme to replace speed monitors on its Airbus planes.

Mr Bouillard told reporters that investigators found "neither traces of fire nor traces of explosives."

He said there was also concern about the length of the delay - six hours - between the crew failing to contact air traffic controllers in Dakar, western Africa, as planned and the alarm being raised.

He said his team was "a long way from establishing the causes of the accident" but that the search for the Airbus A330's data recorders would be extended to 10 July.

The French investigation appears to contradict earlier reports attributed to Brazilian pathologists.

They said last month that the injuries sustained by the passengers whose bodies had been found suggested the plane had been in pieces before it hit the sea.

Mr Bouillard said France had not yet been given access to those autopsy reports.

Search teams have recovered 51 bodies from the ocean but said last month that finding any more remains was "impossible".