Boeing Co., the maker of the grounded 737 Max jet, knew for "some months" about messages between two employees in which one of them expressed serious concerns about the troubled craft, officials said.

But the company delayed handing over the communications to federal regulators investigating the key flight-control system on its jet following two deadly crashes, according to the Federal Aviation Administration.

The communication was an instant message chat between two employees in 2016, according to a copy obtained by NBC News.

Mark Forkner, the Max's chief technical pilot, told a colleague there were problems with the jet and that, "I basically lied to the regulators (unknowingly)."

He went on to describe his own difficulty in handling the craft.

"I'm leveling off at like 4000 ft, 230 knots and the plane is trimming itself like crazy (sic)," Forkner wrote, using the flight term for adjusting aerodynamic forces so that the plane maintains a set altitude. "I'm like WHAT?"

The conversation came before two fatal crashes in 2018-19, that killed more than 300 people.

FAA chief Steve Dickson bluntly demanded an explanation from Boeing.

"I expect your explanation immediately regarding the content of this document and Boeing's delay in disclosing the document to its safety regulator," Dickson wrote in a letter Friday to Boeing President and CEO Dennis Muilenburg.

Late Friday afternoon, Boeing Vice President Gordon Johndroe said in a statement that Muilenburg called Dickson directly "to respond to the concerns raised in his letter."

"Mr. Muilenburg assured the Administrator that we are taking every step possible to safely return the MAX to service," Johndroe added.

Boeing has said it's cooperating with regulators in hopes of getting the 737 Max back in the air someday.

“Over the past several months, Boeing has been voluntarily cooperating with the House Transportation & Infrastructure Committee’s investigation into the 737 MAX," the company said in a statement.

"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."

Reuters was the first to report on the existence of the messages.

"Boeing alerted the Department of Transportation to the existence of instant messages between two Boeing employees, characterizing certain communications with the FAA during the original certification of the 737 MAX in 2016," according to a statement by the FAA.

"Boeing explained to the Department that it had discovered this document some months ago. The Department immediately brought this document to the attention of both FAA leadership and the Department’s Inspector General. The FAA finds the substance of the document concerning. 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."

Two fatal crashes of 737 Max jets have called the craft’s stability into question.

Lion Air Flight 610 went down Oct. 29, 2018, after it took off from Jakarta, Indonesia, killing all 189 people on board.

Then on March 10, Ethiopian Airlines Flight 302 crashed just after takeoff from Addis Ababa, Ethiopia. All 157 people onboard were killed.