Officials with knowledge of the investigation and independent analysts all stressed they don't know why a plane that seemed to be flying normally crashed just minutes after the pilot messaged that he was entering an area of extremely dangerous storms.
They will have little to go on until they recover the plane's "black box" flight data and voice recorders, now likely on the ocean floor miles beneath the surface.
Other hypotheses - even terrorism - haven't been ruled out, though there are no signs of a bomb. Officials have said a jet fuel slick on the ocean's surface suggests there was no explosion.
Two officials told The Associated Press that investigators are looking at the possibility an external probe that measures air pressure may have iced over. The probe feeds data used to calculate air speed and altitude to onboard computers. Another possibility is that sensors inside the aircraft reading the data malfunctioned.
If the instruments were not reporting accurate information, the jet could have been traveling too fast or too slow as it entered turbulence from towering bands of thunderstorms, according to the officials.
"There is increasing attention being paid to the external probes and the possibility they iced over in the unusual atmospheric conditions experienced by the Air France flight," one of the industry officials explained to the AP, speaking on condition of anonymity because he isn't authorized to discuss the investigation publicly.
Meteorologists said the Air France jet entered an unusual storm with 100 mph updrafts that acted as a vacuum, sucking water up from the ocean. The incredibly moist air rushed up to the plane's high altitude, where it quickly froze in minus-40 degree temperatures. The updrafts also would have created dangerous turbulence.
The jetliner's computer systems ultimately failed, and the plane broke apart as it crashed into the Atlantic on a flight from Rio de Janeiro to Paris Sunday night.
Independent aviation experts said it is plausible that a problem with the external probe - called a "pitot tube" - or sensors that analyze data collected by the tube could have contributed to the disaster.
The tubes have heating systems to prevent icing. But if those systems somehow malfunctioned, the tubes could quickly freeze at high altitude in storm conditions, said the other industry official, who also was not authorized to discuss the investigation.
Other experts outside the investigation said it is more likely that the sensors reading information from the tubes failed.
"When you have multiple system failures, sensors are one of the first things you want to look at," said John Cox, a Washington-based aviation safety consultant and former crash investigator for the Air Line Pilots Association.
Jetliners need to be flying at just the right speed when encountering violent weather, experts say - too fast and they run the risk of breaking apart. Too slow, and they could lose control.
"It's critical when dealing with these conditions of turbulence to maintain an appropriate speed to maintain control of the aircraft, while at the same time not over-stressing the aircraft," said Bill Voss, president and CEO of the Flight Safety Foundation in Alexandria, Va.
France's accident investigation agency has established that the series of automatic messages gave conflicting signals about the plane's speed, and that the flight path went through dangerously stormy weather. The agency warned against any "hasty interpretation or speculation" after the French newspaper Le Monde reported, without naming sources, that the Air France plane was flying at the wrong speed.
Two buoys - standard emergency equipment on planes - were spotted Thursday in the Atlantic Ocean about 340 miles (550 kilometers) northeast of Brazil's northern Fernando de Noronha islands by a helicopter crew, which was working off a Brazilian navy ship.
Among other debris spotted were a 23-foot (seven-meter) chunk of plane, an airline seat, an oil slick and several large brown and yellow pieces that probably came from inside the plane, said Brazilian Air Force Gen. Ramon Cardoso. But a wooden cargo pallet picked up Thursday turned out not to be from Flight 447. The plane was not carrying wooden pallets, Cardoso said.
Air France's CEO Pierre-Henri Gourgeon told family members at a private meeting that the plane disintegrated, either in the air or when it slammed into the ocean, and there were no survivors, according to Guillaume Denoix de Saint-Marc, a grief counselor who was asked by Paris prosecutors to help counsel relatives.
More than 500 people packed the historic Candelaria church in the center of Rio de Janeiro Thursday for a Mass for the victims.
With the crucial flight recorders still missing, investigators were relying heavily on the plane's automated messages to help reconstruct what happened as the jet flew through thunderstorms.
The last message from the pilot was a manual signal at 11 p.m. local time Sunday saying he was flying through an area of black, electrically charged cumulonimbus clouds that come with violent winds and lightning.
At 11:10 p.m., a cascade of problems began: the autopilot had disengaged, a key computer system switched to alternative power, and controls needed to keep the plane stable had been damaged. An alarm sounded indicating the deterioration of flight systems. Then, systems for monitoring air speed, altitude and direction failed. Controls over the main flight computer and wing spoilers failed as well. At 11:14 p.m., a final automatic message signaled loss of cabin pressure and complete electrical failure as the plane was breaking apart.
The pilot of a Spanish airliner flying nearby at the time reported seeing a bright flash of white light plunging to the ocean, said Angel del Rio, spokesman for the Spanish airline Air Comet.
The pilot of the Spanish plane, en route from Lima, Peru to Madrid, said he heard no emergency calls.
France's defense minister and the Pentagon have said there were no signs that terrorism was involved.
"We have no evidence, we have no proof, we don't know," French Foreign Minister Bernard Kochner said after he was asked about the possibility of a bomb. "Is it possible? I mean to look at an explosion? Yes it is. It is one of the hypotheses."
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Lowy reported from Washington. Associated Press writers Alan Clendenning and Stan Lehman in Sao Paulo; Marco Sibaja in Brasilia; Slobodan Lekic in Brussels; Daniel Woolls in Madrid; and Greg Keller, Angela Charlton and Emma Vandore in Paris also contributed to this report.
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