Investigators looking into the crash of AirAsia Flight QZ8501 believed that the co-pilot was at the controls before the jet went into a dangerously fast climb, it was reported.
Indonesian authorities are looking into "what factors may have surprised or confused the first officer - who was much less experienced than the captain - and caused the nose of the Airbus A320 to point upward at an unusually steep angle while the plane's computerised stall-protection systems either malfunctioned or were disengaged," the Wall Street Journal (WSJ) on Thursday quoted two sources familiar with the probe as saying.
The plane lost forward airspeed during its rapid climb, stalled and then crashed into the water below, the report said.
The sources told WSJ that investigators believe First Officer Rémi-Emmanuel Plesel, a French national, was flying the aircraft as it manoeuvred to avoid a storm 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 are examining the interaction of pilot commands and computer-controlled flight systems during the climb and subsequent descent, the report said.
Repeated automated stall warnings were captured on the cockpit voice recorder as the first officer and the captain struggled to regain control of the jet, investigators have said.
Indonesia's National Transportation Safety Committee is to submit its preliminary report on the crash to the International Civil Aviation Organisation and related parties on Thursday, but will not publicise it.
The National Search and Rescue Agency (Basarnas) said it will continue to look for the wreckage and remaining bodies on Saturday, after the military withdrew from joint search and recovery efforts a month after the crash.
By law, operations will be reviewed after seven days if no bodies are found, and Basarnas will have to consult with related parties on whether to continue with the search.