A French judge filed preliminary charges of manslaughter Friday against Air France over a 2009 crash that killed all 228 people aboard a jet that plunged into the Atlantic Ocean.

Air France vigorously protested the move, an unusual step in an emotional investigation into the worst-ever accident for France's No. 1 airline.

Judge Sylvie Zimmerman filed the preliminary charges a day after doing the same against Airbus, the maker of the doomed jet and one of Europe's biggest manufacturers.

Air France Flight 447 dived into the Atlantic on June 1, 2009, amid an intense, high-altitude thunderstorm while flying from Rio de Janeiro to Paris.

The cause of the crash remains unclear, and may never be determined without the "black box" flight recorders, somewhere in the ocean depths. A fourth search operation aimed at looking for them starts next week.

Automatic messages sent by the Airbus 330 jet's computers show it was receiving false air speed readings from sensors known as pitot tubes. Investigators have said the crash was likely caused by a series of problems, and not just sensor error.

"We are protesting this," CEO Pierre-Henri Gourgeon told reporters at the courthouse. "It seems to us that it is unfounded."

"We do not understand and we do not recognize the reason or any good reason to justify the fact that we are prosecuted today," he said.

Under French law, preliminary charges mean the investigating magistrate has sufficient reason to suspect wrongdoing. The step allows the magistrate to continue investigating before determining whether to send the case to trial.

Air France lawyer Fernand Garnaud said the judge did not elaborate on reasons for the move.

Gourgeon said the pitot problems were "a contributing factor but not the principal cause" of the crash. He said Air France had taken all necessary measures to fix faulty sensors.

Airbus knew since at least 2002 about the pitot problems, but air safety authorities did not order their replacement until after the crash.

The tubes, about the size of an adult hand and fitted to the underbelly of a plane, are vulnerable to blockage from water and icing. Experts have suggested that Flight 447's sensors, made by French company Thales SA, may have iced over and sent false speed information to the computers as the plane ran into a thunderstorm at about 35,000 feet (10,600 meters).

In November, Air France issued a memo to investigators saying the carrier had counted 15 incidents in which the sensors had iced over on the same aircraft type in the 10 previous months before the crash.

The airline said it had informed Airbus and Thales about those findings, and estimated that about 16 documents that traced Air France's exchanges with Airbus showed that the planemaker didn't respond to its concerns.

Air France and Airbus will finance the estimated US$12.5 million cost of the new search effort, in which three advanced underwater robots will scour the mountainous ocean floor between Brazil and western Africa, in depths of up to 4,000 meters (13,120 feet).

Gourgeon said Friday that the specialists in charge "feel convinced that they have a chance of finding some elements, of finding the wreckage, and if everything happens in ideal conditions, we might even envisage finding this famous black box."