A senior pilot at Boeing said he might have unintentionally misled regulators, according to a series of internal company messages that were released Friday.

The revelation of the messages came as Boeing continues to struggle with the fallout from two fatal crashes that have grounded its 737 Max airplanes.

The transcript of the messages shows that in 2016 the 737 Max's then-chief technical pilot, Mark Forkner, told a co-worker that the aircraft's flight system, called MCAS, was "egregious" and "running rampant" while he tested it in a flight simulator.

The MCAS system has been tied to the crashes of the 737 Max airplanes in Indonesia and Ethiopia.

Forkner said in one text message, "I basically lied to the regulators [unknowingly]."

Boeing revealed messages

Boeing provided the internal messages to lawmakers, who are holding hearings this month on the 737 Max airplanes.

The Federal Aviation Administration (FAA) called the newly released document "concerning" and demanded an explanation about why the company delayed before revealing the messages.

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

Muilenburg, who was stripped of his chairman title by Boeing's board last week, is scheduled to testify before Congress this month.

Forkner left Boeing last year and joined Southwest Airlines - the largest operator of the Boeing 737.

Forkner's lawyer, David Gerger, said in a statement, "If you read the whole chat, it is obvious that there was no 'lie.' " He said Forkner's messages showed that the pilot thought the flight simulator was not working and "absolutely thought this plane was safe."

Two fatal crashes

An Ethiopia Airlines 737 Max crashed just after takeoff in March, killing all 157 people on board. Five months earlier, the same type of plane flown by the Indonesian airline Lion Air crashed shortly after takeoff, killing 189 people.

Investigators have focused on the MCAS system in the planes, a new automated flight system that was not included in previous versions of the 737. Investigators believe a faulty sensor in the MCAS system pushed the nose of each plane down and made it impossible for the pilots to regain control.