Search teams have recovered more than 400 pieces of debris from Air France Flight 447 but investigators still do not know why the plane crashed into the Atlantic, the French accident chief said Wednesday.

Paul-Louis Arslanian, head of the French air accident investigation agency BEA, expressed "a little more optimism" about the investigation as the discovery of so much debris has narrowed the vast search zone off the northeast coast of Brazil.

"We are in a situation that is a bit more favorable than the first days," Arslanian told a news conference at BEA headquarters by the Le Bourget air field outside Paris. "We can say there is a little less uncertainty, so there is a little more optimism."

"[But] it is premature for the time being to say what happened," he added.

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Rescuers and military search equipment from Brazil, France, the United States and other countries are methodically scanning the surface and depths of the Atlantic for signs of the Airbus A330 that crashed May 31 after running into thunderstorms en route from Rio de Janeiro to Paris. All 228 people aboard were killed.

Arslanian said the debris that has been found came from "all zones" of the plane, but did not describe it in detail or say what proportion of the plane had been retrieved. The wreckage, some in sections so large and heavy that cranes are required to move it, is being collected in a hangar in Recife, Brazil.

"It is almost certain today that the whole plane won't be recovered," he said.

Still missing are the plane's two black boxes, its flight data and voice recorders, thought to be deep under water. The black boxes, which provide information about what happened to the plane before and during the crash, will emit signals for at least another two weeks. After that, the signals will fade.

French-chartered ships are trolling with two high-tech U.S. Navy underwater listening devices attached to 19,700 feet (6,000 meters) of cable. The black boxes send out an electronic tapping sound that can be heard up to 1.25 miles (2 kilometers) away.

That search area has a radius of 50 miles (80 kilometers), Arslanian said.

"It is one of the worst situations ever known in an accident investigation," Arslanian said.

Without the black boxes, the investigation so far has focused on a flurry of automated messages sent by the plane minutes before it lost contact. One of those messages suggested that external speed sensors had iced over and destabilized the plane's control systems.

Arslanian said most of the messages "appeared as linked to this loss of validity of speed information." He said when the speed information became "incoherent" it affected other systems on the plane that relied on that speed data. But he stressed that not all the automated messages were related to the speed sensors.

The automated messages were not alarm calls and no distress call was picked up from the plane, he said.

Air France has replaced the sensors, called Pitot tubes, on all its A330 and A340 aircraft, under pressure from pilots who feared a link to the accident.

He said French doctor from BEA was not allowed to participate in the autopsies done so far on some Flight 447 bodies by Brazilian authorities, and those autopsy results have not been released to French investigators. He said he was "not happy" with this situation.

"Eventually I'll have I hope an explanation," Arslanian said.

With another body found Tuesday, 50 have been recovered so far.

French ships also retrieved a few bodies and wreckage but they are still in the search zone, Arslanian said. He said it would be difficult to send those ships to shore because they are needed for the rescue operation, and therefore it was unclear where those bodies would be autopsied.

That wreckage will be brought east, possibly to Dakar, Senegal, he said.

The French are leading the crash investigation, while the Brazilians are leading the rescue operation.