Boeing pilots discussed 'fundamental issues' with 737 MAX in internal messages

A copy of instant messages obtained by CNN from a congressional source show one Boeing pilot saying the MCAS stabilization system was "running rampant" in simulator testing. He wrote the plane had "some real fundamental issues that they claim they're aware of.

"I basically lied to the regulators (unknowingly)," the employee wrote.

His colleague responded: "It wasnt a lie, no one told us that was the case."

The 737 MAX remains grounded and airlines have canceled flights into next year. Boeing has missed its targets for completing and proving the efficacy of safety fixes to the plane following two crashes that killed 346 people.

Reuters first reported on the messages.

The documents were given to the US Transportation Department on Thursday night, Federal Aviation Administration chief Stephen Dickson wrote to Boeing CEO Dennis Muilenburg on Friday. Dickson requested an "explanation immediately" from Boeing's CEO "regarding the content of this document and Boeing's delay in disclosing the document to its safety regulator."

Boeing described the document as "containing statements by a former Boeing employee" but did not explain its delay in sharing the information with the FAA.

The FAA said in a statement accompanying the letter that it has shared the information with congressional committees and the US Department of Transportation inspector general -- who are conducting two of several wide-ranging investigations and reviews into the MAX and FAA's practices.

"We will continue to cooperate with the Committee as it continues its investigation," Boeing spokesman Charles Bickers said in a statement.

It is unclear whether the documents could impact the timeline for regulators' review of the plane. Airline pilots are scheduled to begin FAA-organized simulator testing in November of Boeing's software fixes, according to two sources familiar with the plans.

This story is breaking and will be updated.