Signals sent by Air France Flight 447 before it disappeared show its autopilot was not on, the head of the French agency leading the investigation into the crash of 447 said Saturday.

Agency head Paul-Louis Arslanian said it was not clear if the autopilot had been switched off by the pilots or had stopped working because it received conflicting airspeed readings.

Plane manufacturer Airbus says the investigation found the flight received inconsistent readings from different instruments as it struggled in a massive thunderstorm.

Alain Bouillard, head of the investigation into the crash, told reporters that, "we also saw messages that show the automatic pilot wasn't working."

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The agency says Air France had not replaced instruments that measure air speed on the plane, which the manufacturer had recommended.

Arslanian says some problems had been detected with the instruments on the Airbus A330, the model that disappeared over the Atlantic Ocean on May 31.

Arslanian says Airbus had recommended that airlines replace the instruments on the A330. The head of the crash investigation says Air France had not changed the instruments known as Pitot tubes on the plane that crashed.

Arslanian warned on Saturday against jumping to conclusions.

He says planes can be flown safely "with damaged systems."

Arslanian said investigators are analyzing 24 messages sent automatically by the plane during the last minutes of the flight.

He said investigators are searching a zone of several hundred square miles for the debris.

It is vital to locate a beacon called a "pinger" that should be attached to the cockpit voice and data recorders, now presumed to be deep in the Atlantic, he said.

"We have no guarantee that the pinger is attached to the recorders," Arslanian said.

Holding up a pinger in the palm of his hand, he said: "This is what we are looking for in the middle of the Atlantic Ocean."

Investigators are trying to determine the location of the debris in the ocean based on the height and speed of the plane at the time the last message was received. Currents could also have scattered debris far along the ocean floor, he said.

"You see the complexity of the problem," he said.

Laurent Kerleguer, an engineer specialized in the ocean floor working with the investigation team, said the zone seen as the most likely site of the debris was 15,112 feet at its deepest point and 2,835 feet at its shallowest.

Water salinity and temperature can affect the distance that the beacon's signal can travel, Kerleguer said.

The Airbus A330 from Rio de Janeiro to Paris disappeared nearly four hours after takeoff on Sunday night, killing all 228 aboard. It was Air France's deadliest plane crash and the world's worst commercial air accident since 2001.