The FAA confirmed on Friday that Boeing told the United States government aviation regulator a day earlier about internal messages that the company had discovered "some months ago" and that characterise "certain communications with the FAA during the original certification of the 737 MAX in 2016".
"The FAA is also disappointed that Boeing did not bring this document to our attention immediately upon its discovery", the agency said in a statement to CNBC. The messages were made while the FAA was certifying the 737 MAX aircraft.
The messages deepened the crisis for the world's largest airplane maker days before Muilenburg, who last Friday was stripped of his chairman title by the board, is due to testify before the Congress on the development of the 737 MAX.
Families of the Indonesia crash victims are in settlement talks with Boeing, while the loved ones of the Ethiopian crash victims are pursuing a trial, CNBC reported Thursday.
Reviews since the crashes have found fault with Boeing and the Federal Aviation Administration (FAA), which regulates flight safety, for inadequate review of the risks associated with a new anti-stall software control system known as Mcas.
"Granted I suck at flying, but even this was egregious", Forkner replied.
The U.S. regulator said Boeing Co. was aware of the communication for months.
He said he had "basically lied to the regulators [unknowingly]".
Boeing shares tumbled 5% on the disclosure of the communications that the FAA had been unaware of. "The agency will lift the grounding order only after we have determined the aircraft is safe".
Among other USA airlines that operate the Max, Southwest Airlines Co has canceled flights through January 5 and American Airlines Group Inc until January 15.
Forkner's lawyer, David Gerger, said that Forkner was indicating in messages to a colleague that the flight simulator was not working like the plane.
The FAA said it flagged its concerns to lawmakers on Capitol Hill and the Department of Transportation's inspector general.
Boeing turned over the transcript to the Justice Department earlier this year gave it to Congress only this week in anticipation of Muilenburg's scheduled October 30 testimony before DeFazio's committee, according to a person familiar with the matter.
Shares of Boeing fell 4 percent when the news broke on Friday.