The black box flight recorders of doomed Air France flight 447 may never be found, a leading accident investigator said today.
The Airbus, carrying 228 people, crashed into one of the deepest parts of the Atlantic hindering the search for clues to the disaster.
The head of France's accident investigation agency Paul-Louis Arslanian said he was "not optimistic" that rescuers will recover the recorders.
Mr Arslanian said if rescuers did not find them investigators should be prepared to continue the probe.
He also said there were no signs of problems with the plane before takeoff and it was unclear whether the chief pilot was at the controls when the plane plunged into the Atlantic Ocean.
The Airbus lost contact with ground controllers after leaving Rio de Janeiro for Paris on Sunday night. The crew made no distress call before the crash, but the plane's system sent an automatic message just before it disappeared, reporting lost cabin pressure and electrical failure.
The reason for the crash remained unclear, with fierce thunderstorms, lightning or a catastrophic combination of causes as possible theories.
Mr Arslanian said investigators were studying the plane's maintenance and other records, and that there were "no elements" suggesting the plane had problems before takeoff.
He stressed the investigation was only beginning and was likely to be long. He said he did not have enough information to determine whether the plane broke up in the air or upon impact with the sea.
"We don't even know the exact time of the accident," he said.
Asked whether the chief pilot was in the cockpit when the plane went down, he said, "We don't have, for the time being, the answer." Pilots on long-haul flights often take turns at the controls to remain alert.
"Until the last minute we all hoped it was just a normal loss of contact. It's very difficult to accept such a disaster," he said.
Military planes and ships from Brazil and France homed today on the wreckage of the Air France jet, 400 miles north-east of the Fernando de Noronha islands off Brazil's northern coast, where an aeroplane seat, a fuel slick and pieces of white debris were spotted yesterday.
Boats from several nations were sailing toward the site to start the recovery.
The man in charge of the investigation, Alain Bouillard, led the probe into the 2000 crash of the Air France Concorde. He said today the French accident investigation agency would submit its first preliminary report by the end of the month.