The lead investigator looking into the fatal crash of AirAsia Flight 8501 said the co-pilot was at the controls before the jet went into a dangerously fast climb.

Mardjono Siswosuwarno, head of Indonesia’s air-crash probe agency, also told reporters on Thursday that the plane was in sound condition and the cockpit crew had proper licenses before the accident.

Indonesian authorities are delving into what factors may have surprised or confused the first officer, who was much less experienced than the captain, according to two safety experts familiar with the probe. Investigators, these people said, seek to determine what caused the nose of the Airbus A320 to point upward at an unusually steep angle, while the plane’s computerized stall-protection systems either malfunctioned or were disengaged.

The Airbus A320 lost forward airspeed during its rapid climb, stalled and then crashed into the water below.

After spending more than two weeks analyzing twin black-box recorders, investigators believe First Officer Rémi-Emmanuel Plesel, a French national born in the Caribbean territory of Martinique, was flying the aircraft as it maneuvered to avoid a storm cell on Dec. 28 en route to Singapore from Surabaya, Indonesia. Turbulence or updrafts are suspected of contributing to the plane’s dramatic climb, but investigators continue to examine the interaction of pilot commands and computer-controlled flight systems during the climb and subsequent descent.

Repeated automated stall warnings were captured on the cockpit-voice recorder as the co-pilot and captain struggled unsuccessfully to regain control of the jet, investigators have said.

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