FAA Administrator Steve Dickson also wrote to Boeing CEO Dennis Muilenburg demanding an "immediate" explanation for why the texts had not been turned over sooner.
Text messages between two Boeing employees implying they "lied to the regulators" about problems with the safety software of 737 MAX passenger jets have been reportedly given to investigators, as the planes remain out of service.
Boeing did not turn the messages over to the FAA until Thursday, federal officials said Friday. He was recently stripped of the title of chairman of the company, though he remains chief executive.
"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", Boeing said.
The messages are between the MAX's then-chief technical pilot, Mark Forkner, and another Boeing pilot, the sources said, and raised questions about the MCAS's performance in the simulator in which he said it was "running rampant".
Forkner has since left Boeing.
The letter said that Boeing was aware of the troubling instant messages, but the company waited months to disclose them. Neither he nor his lawyer responded to requests for comment.
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The FAA said it "finds the substance of the document concerning" and is disappointed that Boeing waited months before bringing them to the agency's attention.
"So I basically lied to the regulators (unknowingly)", read one message.
Boeing is revising the 737 MAX software to add more safeguards and require the MCAS system to receive input from two key sensors.
Boeing shares tumbled 5% on the disclosure of the communications that the FAA had been unaware of.
All 737 MAX planes were grounded after the Ethiopian Airlines crash in March, and remain out of service the world over, pending investigations by United States regulators.
Separately, the U.S. Senate Commerce Committee confirmed it will question Muilenburg at an October 29 hearing, one day before a House of Representatives panel is scheduled to question him.