Investigators are beginning to form "an image that is progressively less fuzzy," Paul-Louis Arslanian, head of the French air accident investigation agency BEA, said Thursday.

The investigation has focused on a flurry of automated messages sent by the plane minutes before it lost contact; one indicates the airplane was receiving inconsistent speed readings.

Debris from the missing Air France flight 447 being recovered from the Atlantic Ocean arrive at the Recife's port June 19, 2009. [Agencies]

Arslanian said most of the messages appear to be "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.

French and US officials have said there were no signs of terrorism, and Brazil's defense minister said the possibility was not considered. But France says it has not been ruled out.

More than 400 pieces of debris have been recovered, Arslanian said earlier this week. He also called the search conditions, far from land in very deep water, "one of the worst situations ever known in an accident investigation."

Autopsies have revealed fractures in the legs, hips and arms of victims, injuries that -- along with the large pieces of wreckage pulled from the Atlantic -- strongly suggest the plane broke up in the air, experts have said.

Gourgeon said the difficulties that had emerged in the exchange of information between representatives of BEA and Brazilian medical authorities conducting autopsies on recovered bodies were being resolved.

French-chartered ships are trolling a search area with a radius of 50 miles (80 kilometers), pulling US 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, but these locator beacons will begin to fade in less than two weeks.