The Air France passenger jet that crashed off the coast of Brazil two years ago stalled before it plunged for 3½ minutes into the Atlantic, French investigators say.

However, the passengers on that doomed Rio de Janeiro-to-Paris flight were probably asleep or nodding off and didn't realize what was going on as the aircraft fell nose-up toward the sea, the director of the French accident investigating bureau BEA said Friday.

The information on the June 1, 2009, disaster was gleaned from the aircraft's flight recorders, found 3,900 metres beneath the Atlantic Ocean in early May.

The report said the emergency on the Airbus 330-203 aircraft began about 2½ hours into the flight with a stall warning, recorded after the captain left the cockpit for a routine rest period, leaving the two co-pilots in charge.

After the plane went into a stall, warnings sounded, the autopilot and autothrust shut off as designed, and the co-pilot not at the controls "tried several times to call the captain back," the BEA report said. The captain returned one minute and 10 seconds later, when the plane had climbed to 38,000 feet.

"During the following seconds, all of the recorded speeds became invalid and the stall warning stopped," the report said, but added that the plane never came out of its aerodynamic stall.

"The airplane was subject to roll oscillations that sometimes reached 40 degrees," the report said. The engines never stopped operating and "always responded to crew commands," the BEA said.

"The pilots never panicked,"  BEA director Jean-Paul Troadec said on RTL radio, adding that they maintained professionalism throughout.

The brief, highly technical report by the BEA contains only selective remarks from the cockpit recorder, offers no analysis and assigns no blame. It also does not answer the key question: What caused the crash?

But several experts familiar with the report said the co-pilot at the controls, at 32 the youngest of the three-man cockpit crew, Cedric Bonin, may have responded incorrectly to the emergency by pointing the nose upward, perhaps because he was confused by incorrect sensor readings.

The plane's external speed sensors, called Pitot tubes, have long been considered a likely culprit in the disaster, with experts suggesting they may have been iced over. And the BEA investigators found that two sets of instruments on the plane gave different speed readings, with the discrepancies lasting less than a minute.

Since the accident, Air France has replaced the speed monitors on all its Airbus A330 and A340 aircraft.

The crash killed 228 people from more than 30 countries.

One Canadian was aboard, Brad Clemes, 49, originally from Guelph, Ont. He and his wife had been living in Belgium.

BEA described its findings as an update and said a more complete report is expected to be released in July.