A top Boeing pilot working on the 737 Max said in messages from 2016 that a new automated system known as MCAS was making the plane difficult to control in flight simulators. The messages suggest that Boeing officials working on the development of the best-selling jet knew of potential issues with the automated system years before the plane was involved in two deadly crashes.

The system is believed to have played a central role in both fatal accidents, using faulty data to send the planes into unrecoverable nose dives shortly after takeoff.

Below is the exchange the pilot, Mark Forkner, had with a colleague in November 2016. (Identifying information, as well as an expletive, has been intentionally obscured.)