Paris  - Three months after the crash of an Air France Airbus A330 in the Atlantic Ocean, investigators still have no idea what caused the accident, the official leading the inquiry said Monday.

"At the moment, we are unable to explain the accident," Paul-Louis Arslanian, the head of the French Office of Accident Investigation (BEA), told journalists in Paris. "We lack an explanation for the loss of 10 kilometres of altitude in four flight minutes. That is enormous."

The plane, carrying 228 people, vanished early on June 1 while on a flight from Rio de Janeiro to Paris, after apparently encountering bad weather. There were no survivors.

Searchers have so far recovered only 51 bodies and about 1,000 fragments of the aircraft. The wreckage of the plane and its two black boxes - a cockpit voice recorder and a flight data recorder, which contain data essential to the investigation - have not been found.

"When we find the wreck, we will have a chance to find the black boxes," Arslanian said.

However, the search area is approximately the size of Switzerland and has a similar terrain, he said, adding that the investigation could last about one-and-a-half years.

Arslanian said the search for the wreck and the black boxes would resume sometime in autumn.

Earlier reports based on the aircraft's final signals suggested that the airspeed sensors, or Pitot tubes, malfunctioned and may have led to the crash.

But Arslanian said Monday that most of the Pitot tube malfunctions lasted only several seconds and that the plane's pilots did not react to them. (dpa)