Boeing anti-stall software on the doomed Ethiopian Airlines jet re-engaged as many as four times before it crashed, after the crew initially turned it off due to suspect data from an airflow sensor.

That's according to two sources familiar with the investigation.

But it was not immediately clear whether the crew had chosen to re-deploy the system, which pushes the nose of the Boeing 737 MAX downwards.

One source said investigators were studying the possibility that the software had kicked in again without human intervention.

A Boeing spokeswoman declined to comment.

And Ethiopian investigators were not immediately available for comment The anti-stall software is at the center of investigations into both the Ethiopian Airlines crash last month and a Lion Air accident in Indonesia in October - both planes were Boeing 737 MAX 8 aircraft.

On Wednesday, The Wall Street Journal reported that the Ethiopian Airlines pilots had initially followed Boeing's emergency procedures.

But still failed to regain control of the plane.

The crash killed all 157 people on board and led to a global grounding of 737 MAX jets.

Boeing had issued guidelines to pilots about shutting off an automated anti-stall system in the wake of the deadly crash in Indonesia less than five months earlier.