"We are in a situation that is a bit more favorable than the first days," Paul-Louis Arslanian, head of the French civil aviation safety agency told reporters at a press conference in Paris on Wednesday. "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 cautioned that the search is continuing for the flight data recorders, which could hold critical clues as to what happened.
Investigators so far have focused on the potential that speed sensors, called Pitot tubes, could have iced up and provided faulty information to the flight control computers. In the past week, Air France has replaced the Pitot tubes on all of its Airbus 330s.
That theory was gleaned from the burst of automated messages about mechanical events sent during the last four minutes of the flight. Most of the messages appear to be linked to "incoherent" speed readings, which then affected other systems of the plane, Mr. Arslanian said.
History of Flight 587 probe
But the report that one of those automated messages also indicated problems with the rudder limiter has renewed concerns first made public during the AA587 crash investigation in 2001.
At the time, a group of American Airlines pilots presented to the NTSB a 68-page dossier documenting incidences of uncommanded rudder movements in the A300 series jets.
The NTSB eventually concluded the cause of the crash was not a computer problem, but the co-pilot over-using the rudder pedal during some wake turbulence.
The animation in this NTSB simulation shows the pilots pushing the rudder pedals abruptly and sharply to the floor, which is what investigators believed caused the plane to lose its vertical stabilizer and crash.
But some pilots familiar with the A300 series jets still doubt that conclusion. They say that it would be physically very difficult for a pilot to make the kind of abrupt rudder pedal movements indicated in the simulation, particularly while going 250 knots, which the NTSB indicated was the plane's speed at the time.