"It is an element but not the cause," Bouillard told a news conference in Le Bourget outside Paris. "Today we are very far from establishing the causes of the accident."
The Airbus A330-200 plane was flying from Rio de Janeiro to Paris when it went down with 228 people on board in a remote area of the Atlantic, 930 miles (1,500 kilometers) off Brazil's mainland and far from radar coverage.
The BEA released its first preliminary findings on the crash Thursday, calling it one of history's most challenging plane crash investigations. Yet the probe, which has operated without access to the plane's flight data and voice recorders, appears to have unveiled little about what really caused the accident.
"Between the surface of the water and 35,000 feet, we don't know what happened," Bouillard admitted. "In the absence of the flight recorders, it is extremely difficult to draw conclusions."
One of the automatic messages emitted by the Air France plane indicates it was receiving incorrect speed information from the external monitoring instruments, which could destabilize the plane's control systems. Experts have suggested those external instruments might have iced over.
The Pitots have not been "excluded form the chain that led to the accident," he said.
Bouillard said the plane "was not destroyed in flight" and appeared to have hit "belly first," gathering speed as it dropped thousands of feet through the air.
He said investigators have found "neither traces of fire nor traces of explosives."
Bouillard said life vests found among the wreckage were not inflated, suggesting that passengers were not prepared for a crash landing in the water. The pilots apparently also did not send any mayday calls.
He said there was "no information" suggesting a need to ground the world's fleet of more than 600 A330 planes as a result of the crash.
"As far as I'm concerned there's no problem flying these aircraft," he said.
A burst of automated messages emitted by the plane before it fell gave rescuers only a vague location to begin their search, which has failed to locate the plane's black boxes in the vast ocean expanse. The chances of finding the flight recorders are falling daily as the signals they emit fade. Without them, the full causes of the tragic accident may never be known.
The black boxes - which are in reality bright orange - are resting somewhere on an underwater mountain range filled with crevasses and rough, uneven terrain. Bouillard said the search for the plane's black boxes has been extended by 10 days and will continue through July 10.
The remote location, combined with the mystery of what happened to the plane - the pilots had either no time or no radio frequency to make a mayday call - makes the inquiry exceptionally challenging.
Bouillard said French investigators have yet to receive any information from Brazilian authorities about the results of the autopsies on the 51 bodies recovered from the site.
Families of the victims met with officials from BEA, Air France and the French transport ministry before the report was released Thursday. An association of families addressed a letter to the CEO of Air France, Pierre-Henri Gourgeon, demanding answers to several questions about the plane.
Investigators should have an easier time recovering debris and black boxes in the crash of a Yemeni Airbus 310 with 153 people on board that went down Tuesday just nine miles (14.5 kilometers) north of the Indian Ocean island-nation of Comoros.
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Vandore reported from Paris. Associated Press writers Cecile Brisson at Le Bourget and Angela Charlton in Paris contributed to this report.
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