French officials said today there were no signs of problems with Air France Flight 447 before takeoff and it was unclear whether the chief pilot was at the controls when the plane plunged into the Atlantic Ocean.
The head of France's accident investigation agency, Paul-Louis Arslanian, also said he was "not optimistic" that rescuers would recover the plane's black boxes miles underwater.
Pilots on long-haul flights often take turns at the controls to remain alert. Asked whether the chief pilot was in the cockpit when the plane went down, Arslanian told a news conference in France that there was no information either way.
"We don't even know the exact time of the accident," he said.
But Arslanian said there was no indication of problems with the plane before it left Rio de Janeiro on Sunday night en route to Paris.
At the search scene Brazilian navy divers rushed to reach the wreckage and start retrieving debris from the Atlantic.
Four navy ships with recovery equipment and a tanker were heading to a three mile (5km) strip of water strewn with plane seats, an orange buoy, wiring, metal pieces and jet fuel stains about 745 miles north-east of the coastal city of Recife.
As the navy battled rough weather, officials warned of the difficulty in finding the flight data and voice recorders – the black boxes – that hold clues to why the plane crashed during a severe storm in the middle of the night.
Officials said the recorders needed to identify the causes of the mysterious crash could be on the ocean floor at a depth of 6,600 to 9,800 feet (2,000 to 3,000 metres).
The recorders are designed to send homing signals for up to 30 days when they hit water.
One expert said it could be among the hardest recoveries since the decades-long search to find the Titanic.
"If you think how long it took to find the Titanic and that the debris would be smaller, you are looking for a needle in haystack," said Derek Clarke, joint managing director of Aberdeen-based Divex, which designs and builds military and commercial diving equipment.
However, the Brazilian president, Luiz Inacio Lula da Silva, said he was confident that the black boxes – two separate devices containing cockpit voice recordings and instrument data – offer the best chance of finding out why the Airbus jetliner vanished. .
"I think a country that can find oil 6,000 metres under the ocean can find a plane 2,000 metres down," he told reporters yesterday in Guatemala, referring to recent oil finds by Brazil's state energy company in ultra-deep waters.
Officials from the US National Transportation Safety Board and from Eurocontrol say that investigators have many ways to begin investigating the accident even before they recover any wreckage or the black boxes.
"Investigators will have to do a forensic analysis, by piecing together all available information as best they can," said Jim Hall, a former chairman of the NTSB.
They will review the maintenance records of the aircraft, interview the crews who flew the plane in the last few weeks and go to the locations where recent maintenance was done to interview mechanics.
They will also study the personal histories of the crew members and reconstruct what they did in the last 36 hours before the crash.
"In other words, they'll be compiling as much background information as they can to compensate for the lack of other data," Hall said.