LE BOURGET, France — Air France Flight 447 slammed into the Atlantic Ocean, intact and belly first, at such a high speed that the 228 people aboard probably had no time to even inflate their life jackets, French investigators said Thursday in their first report into the June 1 accident.

Likening the investigation to a puzzle with missing pieces, lead investigator Alain Bouillard said that one month after the crash, "we are very far from establishing the causes of the accident."

Problematic speed sensors on the Airbus A330-200 jet that have been the focus of intense speculation since the crash may have misled the plane's pilots but were not a direct cause, Bouillard said, while admitting that investigators are still a long way from knowing what did precipitate the disaster.

"The investigation is a big puzzle," said Bouillard, who is leading the probe for the French accident agency BEA. "Today we only have a few pieces of the puzzle, which prevents us from even distinguishing the photo of the puzzle."

The plane was flying from Rio de Janeiro to Paris when it went down in a remote area of the Atlantic, 930 miles 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 so far to have unveiled little about what really caused the accident.

The speed sensors, called Pitot tubes, are "a factor but not the only one," Bouillard said. "It is an element but not the cause," he told a news conference in Le Bourget outside Paris.

Other elements that came under scrutiny in the immediate aftermath of the crash, such as the possibility that heavy storms or lightning may have brought down the jet, were also downplayed in the BEA's presentation.

Meteorological data show the presence of storm clouds in the area the jet would have flown through, but nothing out of the ordinary for the equatorial region in June, Bouillard said, eliminating the theory that the plane could have encountered a storm of unprecedented power. Other flights through the area shortly after Flight 447 disappeared didn't report unusual weather, Bouillard said.

"Between the surface of the water and 35,000 feet, we don't know what happened," Bouillard acknowledged. "In the absence of the flight recorders, it is extremely difficult to draw conclusions."