KEY POINTS
  • The FAA said Boeing withheld the messages between employees for months, sending shares lower.
  • The incident ramps up pressure on Boeing and CEO Muilenburg about the planes.
  • Boeing and the FAA are facing several investigations about the 737 Max's design and certification.

The Federal Aviation Administration said Boeing withheld "concerning" messages from 2016 between employees about a flight-control system implicated in two crashes of the 737 Max, deepening the manufacturer's crisis over the jets that have been grounded worldwide since March.

A Boeing test pilot complained in one of the messages that a flight control system, known as MCAS, was difficult to control, according to the messages, obtained by NBC News.