RIO DE JANEIRO - Investigators trying to determine why Air France Flight 447 broke apart in a violent storm over the Atlantic are looking at the possibility that speed sensors - or an external instrument key to collecting speed data - failed in unusual weather, two aviation-industry officials said yesterday.

Meanwhile, Brazil's navy issued a statement saying that wreckage recovered by a helicopter crew earlier in the day was not from the plane. The military earlier said it had pulled a cargo pallet from the water where the Airbus A330 went down off the country's northeastern coast, killing all 228 people aboard - the world's worst aviation disaster since 2001.

Officials with knowledge of the investigation and independent analysts all stressed they do not 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.

Other hypotheses - even terrorism - have not been ruled out, though there are no signs of a bomb. Officials have said a jet-fuel slick on the ocean's surface suggested there was no explosion.

Two officials with knowledge of the investigation said they were 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 that read the data malfunctioned.

If the instruments were not accurately reporting information, it is possible the jet would 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 Associated Press, speaking on condition of anonymity because he is not authorized to discuss the investigation publicly.

Meteorologists said the Air France jet entered an unusual storm with 100-m.p.h. 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 jet's computer systems ultimately failed, and the plane broke apart on a flight from Rio de Janeiro to Paris on Sunday night.

Independent aviation experts said it was 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 crash.

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 was 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, who heads the Flight Safety Foundation in Alexandria, Va.