Jakarta (CNN) The doomed Lion Air flight that crashed into the sea, killing 189 passengers last month, was found to have a malfunctioning air speed indicator for its last four flights -- and, crucially, at the time of the crash, according to the head of Indonesia's National Transportation Safety Committee.

Investigators who analyzed the flight data recorder after the October 29 crash uncovered the fault, the committee said. The finding is the first technical problem revealed as part of the investigation into why the plane crashed.

"We are formulating, with NTSB and Boeing, detailed inspections regarding the airspeed indicator," according to Soerjanto Tjahjono, head of the committee.

"We found a malfunction in an air speed indicator instrument in the last four flights, including the crash flight," Capt. Nurcahyo Utomo of the National Transportation Safety Committee (KNKT) told CNN on Monday. "We ask NTSB and Boeing to work on this to prevent same accident happening in the future."

Utomo confirmed to CNN that there were two airspeed indicators in the cockpit. "One belongs to the pilot in command and one belongs to the co-pilot. One of them should have reacted to the airspeed indicator malfunction in his display," said Nurcahyo, confirming that the jet was in the pilot's control, set to manual.

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