Washington(CNN) Federal regulators on Friday demanded Boeing explain why it withheld documentation of employees' concerns with a software system which investigators have linked to two fatal crashes of the 737 MAX.
The instant messages, obtained by CNN from a congressional source, show internal concerns that the MCAS stabilization system was "running rampant" and more powerful than the company had told the Federal Aviation Administration.
"[T]he plane is trimming itself like craxy," one pilot wrote in a message, several of which contain misspellings, later adding, "granted, I suck at flying, but even this was egregious."
He wrote the plane had "some real fundamental issues that they claim they're aware of."
"I basically lied to the regulators (unknowingly)," the pilot wrote.
His colleague responded: "It wasnt a lie, no one told us that was the case."
In the messages, the pilot refers to his job as "insane" and commiserates with the colleague about internal pressure and feeling out of the loop on developments such as MCAS.
"I'd ask for a job in sales where I can just get paid to drink with customers and lie about how awesome our airplanes are," he wrote.
About four months after these messages are timestamped, the Federal Aviation Administration would approve the 737 MAX for flight, and the MCAS system was left out of the pilot manual.
Those decisions by Boeing and the FAA are now under scrutiny in multiple investigations and reviews.
A copy of the instant messages was obtained by CNN from a congressional source. Reuters first reported on the messages.
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.
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.
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."
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.
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.
"We will continue to cooperate with the Committee as it continues its investigation," Boeing spokesman Charles Bickers said in a statement. Muilenburg spoke to Dickson by phone as well.
Rep. Peter DeFazio, chairman of the House Transportation Committee, said the document reflects "production pressures and a lack of candor with regulators and customers."
This story has been updated.