DALLAS (AP) — A Boeing pilot told a co-worker that he unknowingly misled safety regulators about a flight-control system on the now-grounded 737 MAX, according to the transcript of instant messages that the company belatedly turned over to federal officials.

The pilot, Mark Forkner, told another Boeing employee about problems with the flight system, known as MCAS, during a session in a flight simulator.

"So I basically lied to the regulators (unknowingly)," Forkner wrote in a message from 2016.

MCAS was designed at least in part to prevent the Max from stalling in some situations. After the Federal Aviation Administration certified the plane, without a complete understanding of MCAS, the system was implicated in two crashes that killed 346 people.

Forkner had asked FAA about removing mention of MCAS from the pilot's manual for the Max. FAA allowed Boeing to do so, and most pilots did not know about MCAS until after the first crash in Indonesia, in October 2018. The plane has been grounded worldwide since March after the second crash, in Ethiopia.

On Friday, FAA chief Steve Dickson demanded an explanation from Boeing CEO Dennis Muilenburg, including why the company delayed several months before telling FAA about the messages.

Boeing, in a prepared statement, said the transcript contained the communications of a former employee. Although Boeing didn't identify Forkner, he left several months ago and joined Southwest Airlines — the biggest operator of the Boeing 737. The Associated Press was not able to contact him immediately.

Boeing didn't explain why it waited months before turning over the document, but said it was cooperating with the congressional investigation into the Max.

"The FAA finds the substance of the document concerning," the regulator said in a statement. "The FAA is also disappointed that Boeing did not bring this document to our attention immediately upon its discovery. The FAA is reviewing this information to determine what action is appropriate."

In his terse, three-sentence letter to the Boeing CEO, Dickson wrote, "I expect your explanation immediately regarding the content of this document and Boeing's delay in disclosing the document to its safety regulator."

The disclosure of the internal Boeing communications comes just a week after international regulators faulted the company for not doing more to keep FAA informed about MCAS, a new automated flight system not included in previous versions of the 737.

Before crashes in Indonesia and Ethiopia, MCAS was activated by a faulty sensor and pushed the nose of each plane down. Pilots were unable to regain control. The company is updating software and computers to make the nose-down command less powerful and easier for pilots to overcome.

Boeing shares tumbled 5% on the disclosure of the communications that the FAA had been unaware of.