Tatang Kurniadi (L), chief of the National Transportation Safety Committee (NTSC) holds a model plane during a news conference in Jakarta January 29, 2015.[Photo/Agencies]

JAKARTA - The French first officer of an AirAsia passenger jet that crashed into the sea last month was at the controls just prior to the accident, Indonesia's lead investigator said on Thursday.

Data from the black box flight data recorder has provided the accident probe with a "pretty clear picture" of what happened in the last moments of AirAsia Flight QZ8501, but officials offered few details.

The Airbus A320 vanished from radar screens in bad weather on Dec. 28, less than halfway into a two-hour flight from Indonesia's second-biggest city of Surabaya to Singapore. All 162 people on board were killed.

"The second-in-command, popularly known as the co-pilot, who usually sits to the right of the cockpit. At the time, he was flying the plane," said head National Transport Safety Committee (NTSC) investigator Mardjono Siswosuwarno, referring to first officer Remi Plesel.

"The captain, sitting to the left, was the pilot monitoring."

Captain Iriyanto, 53, was believed to have taken over control of the aircraft from Plesel when it started to ascend and then descend sharply, officials said.

The cause of AirAsia's first fatal crash, which occurred around 40 minutes into the flight, was still unknown.