The Air France plane that vanished in the middle of the Atlantic with 228 people on board did not disintegrate in mid-air but plunged into the water intact and belly first, investigators said today.

Alain Bouillard, leading the preliminary inquiry on behalf of France's BEA accident agency, said examination of wreckage indicated the A330 Airbus was still in one piece when it crashed, at high speed, into the ocean.

"The plane was not destroyed while it was in flight," he told a press conference near Paris. "It seems to have hit the surface of the water in the direction of flight and with a strong vertical acceleration." Appearing to rule out any question ofterrorism, Bouillard added that "neither traces of fire nor traces of explosives" had been found.

Just over a month since Flight AF447 went down during a flightbetween Rio de Janeiro and Paris,, killing all passengers and crew, investigators said they were facing one of the most challenging and baffling cases in the history of air travel.

The pilots apparently sent no distress calls before the plane went missing, and a rescue team has been unable to find the flight recorders, or black boxes, in one of the remotest parts of the Atlantic, 930 miles off Brazil's mainland. Investigators have warned that, without such crucial information, a full explanation into why the Airbus ran into difficulties will be hard to come by.

"Today we are very far from establishing the causes of the accident," admitted Bouillard, adding that the blame for the crash could not be pinned entirely on a problem with the plane's speed sensors, or pitot tubes. "[It] is one of the factors but not the only one," he said. "It is an element but it is not the cause."

The BEA said it was trying to piece together what went wrong from the automated messages, or Acars, sent in the final minutes before the plane hit the water, and from the debris in the Atlantic in the past month.

Around 640 items of furniture, machinery and other material has been examined for clues. Analysis of food trays and shelving in the galley indicated that they had crashed in a way that would suggest a strong vertical acceleration, he said. Part of the plane's floor had been found misshapen to suggest a similar fate.

No inflated life jackets had been discovered, which "obviously shows the passengers were not prepared for a crash landing," said Bouillard. But experts expressed doubt as to where the investigation, still in its early days, could lead without the recovery of the black boxes. Bouillard announced the search for the recorders had been extended for another 10 days in the hope that the equipment would continue to emit signals.

Airbus said it was exploring ways to "reinforce" flight data recovery, either by increasing the data sent from planes, or by developing technology such as black boxes that float or whose signals last longer.

"Without finding the black boxes it's going to be phenomenally difficult, maybe impossible, to determine what happened," Kieran Daly, editor of Air Transport Intelligence, told the Associated Press, adding there was a "horrendous lack of evidence".

Initial rumours surrounding the plane's airspeed sensors was partially corroborated by investigators yesterday, who confirmed that one of the messages transmitted minutes before the crash showed the pilots were trying to fly through a storm zone with faulty speed information.

Speculation that the monitoring instruments, located outside the plane, could have iced over led Air France to replace the monitors on all its Airbus 330s and A340s this month. But, though a symptom of something wrong, investigators stressed they were at a loss to explain the root cause of the crash. "Between the surface of the water and 35,000 feet, we don't know what happened," Bouillard said. "In the absence of the flight recorders, it is extremely difficult to draw conclusions."

Aside from problems with the speed sensors, investigators are also focusing on why air traffic controllers in Brazil failed to pass over control of the flight to their colleagues in Senegal.