A Boeing pilot behind the 737 MAX certification in 2016 told a colleague a key flight handling system was "running rampant" during simulator tests, according to documents released Friday by congressional investigators.

The Boeing employees quipped about problems with the Maneuvering Characteristics Augmentation System (MCAS), a flight-handling mechanism that is believed to be at the center of two MAX crashes that killed 346 people.

Boeing's chief technical pilot of the 737, Mark Forkner, said the MCAS system's performance during the simulator tests was "egregious" and that "I basically lied to the regulators (unknowingly)."

The messages raised fresh questions about Boeing's knowledge of problems with MAX, sending shares sharply lower Friday.

The crashes and the Federal Aviation Administration's certification of the MAX are under investigation from a number of authorities, including the Department of Justice and congressional committees that have scheduled hearings with Boeing Chief Executive Dennis Muilenburg later this month.

The FAA blasted Boeing's handling of the message, saying the aviation giant discovered instant messages between the employees "some months ago" but did not disclose their existence until Thursday.

The FAA said it flagged its concerns to lawmakers on Capitol Hill and the Department of Transportation's inspector general.

"Last night, I reviewed a concerning document that Boeing provided late yesterday to the Department of Transportation," FAA Administrator Steve Dickson said a letter to Boeing chief Dennis Muilenburg.

"I understand that Boeing discovered the document in its files months ago. I expect your explanation immediately regarding the content of this document and Boeing's delay in disclosing the document to its safety regulator."

A Boeing spokesman said the document was shared with congressional investors. "Over the past several months, Boeing has been voluntarily cooperating with the House Transportation & Infrastructure Committee's investigation into the 737 MAX," the Boeing spokesman said.

"As part of that cooperation, today we brought to the committee's attention a document containing statements by a former Boeing employee. We will continue to cooperate with the committee as it continues its investigation. And we will continue to follow the direction of the FAA and other global regulators, as we work to safely return the 737 MAX to service."

News of the messages come as the FAA has taken the lead among international regulators in overseeing the re-certification for the MAX in a process that has dragged on much longer than originally expected.