Autopsies have revealed fractures in the legs, hips and arms of Air France Flight 447 victims, injuries that — along with the large pieces of wreckage pulled from the Atlantic — strongly suggest the plane broke up in the air, experts say.

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With more than 400 pieces of debris recovered from the ocean's surface, the top French investigator expressed optimism about eventually discovering what brought down the plane. But he also called the search conditions — far from land in very deep water — "one of the worst situations ever known in an accident investigation."

French investigators are beginning to form "an image that is progressively less fuzzy," Paul-Louis Arslanian, who runs the French air accident investigation agency BEA, told a news conference Wednesday outside Paris.

"We are in a situation that is a bit more favorable than the first days," Arslanian said. "We can say there is a little less uncertainty, so there is a little more optimism. ... (but) it is premature for the time being to say what happened."

A spokesman for Brazilian medical examiners told The Associated Press on Wednesday that fractures were found in autopsies on an undisclosed number of the 50 bodies recovered so far. The official spoke on condition he not be named due to department rules.

"Typically, if you see intact bodies and multiple fractures — arm, leg, hip fractures — it's a good indicator of a midflight break up," said Frank Ciacco, a former forensic expert at the U.S. National Transportation Safety Board. "Especially if you're seeing large pieces of aircraft as well."

The pattern of fractures was first reported yesterday by Brazil's O Estado de S. Paulo newspaper, which cited unnamed investigators. The paper also reported that some victims were found with little or no clothing, and had no signs of burns.

"In an in-air break up like we are supposing here, the clothes are just torn away," said Jack Casey, an aviation safety consultant in Washington, D.C. and a former accident investigator.

Casey also said multiple fractures are consistent with a midair breakup of the plane, which was cruising at about 34,500 feet when it went down.

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"Getting ejected into that kind of windstream is like hitting a brick wall — even if they stay in their seats, it is a crushing effect," Casey said.

When a jet crashes into water mostly intact — such as the Egypt Air plane that hit the Atlantic Ocean after taking off from New York in 1999 — debris and bodies are generally broken into small pieces, Ciacco said.

Lack of burn evidence would not necessarily rule out an explosion, said John Goglia, a former member of the U.S. National Transportation Safety Board.

Searchers from Brazil, France, the United States and other countries are methodically scanning the surface and depths of the Atlantic for signs of the Airbus A330 that crashed May 31 after running into thunderstorms en route from Rio de Janeiro to Paris. All 228 people aboard were killed.

Brazilian Air Force Col. Henry Munhoz told reporters Wednesday that several more body parts, as well as pieces of the plane and luggage, were found in the search area by the French amphibian ship Mistral.

Still missing are the plane's flight data and voice recorders, thought to be deep under water.

French-chartered ships are trolling a search area with a radius of 50 miles, pulling US Navy underwater listening devices attached to 19,700 feet of cable. The black boxes send out an electronic tapping sound that can be heard up to 1.25 miles away.

U.S. Air Force Col. Willie Berges, commander of the American military forces supporting the search, said the black boxes emit beacons at a unique frequency, virtually guaranteeing that any signal detected would be from the pingers.

"The question becomes if the black box is with the pinger, because they can get separated," Berges said.

Without the black boxes to help explain what went wrong, the investigation has focused on a flurry of automated messages sent by the plane minutes before it lost contact; one suggests external speed sensors had iced over, destabilizing the plane's control systems.

Arslanian said most of the messages appear to be "linked to this loss of validity of speed information." He said when the speed information became "incoherent" it affected other systems on the plane.

The automated messages were not alarm calls and no distress call was picked up from the plane, he said.

Air France has replaced the sensors, called Pitot tubes, on all its A330 and A340 aircraft, under pressure from pilots who feared a link to the accident.