The F.A.A., whose relationship with Mr. Muilenburg had grown increasingly tense during the Max crisis, said on Monday that it did not comment on personnel decisions, but that it was continuing work on approving the plane and had “set no time frame for when the work will be completed.”

A spokesman for the agency said it expected that Boeing would “support that process by focusing on the quality and timeliness of data submittals for F.A.A. review, as well as being transparent in its relationship with the F.A.A. as safety regulator.”

In a note sent to employees on Monday, Mr. Smith, the interim chief executive, pledged “full transparency, including effective and proactive communications with the F.A.A., other global regulators and our customers.”

“This has obviously been a difficult time for our company,” Mr. Smith added in the note, a copy of which was reviewed by The New York Times. “I am committed to ensuring above all that we meet the needs of our stakeholders — especially our regulators, customers and employees — with transparency and humility.”

Mr. Muilenburg is an engineer who had spent his entire career at Boeing, rising to become chief executive in 2015. He spent much of last week at Boeing’s headquarters in Chicago, taking calls and attending meetings related to the Max crisis and the company’s decision to shut down the Max factory. Though executives viewed the production halt as a prudent step, it sent Boeing’s stock down and rippled through the national economy.

Days after that decision, the F.A.A. became aware of more potentially damaging messages from Boeing employees that the company had not turned over to the agency, further straining the company’s relationship with the regulator. Boeing waited months this year to disclose to the F.A.A. that it had found messages from 2016 in which a Boeing pilot complained that MCAS, which was new to the Max, was acting unpredictably in a flight simulator.

On Friday morning, Mr. Muilenburg was in Florida to observe the predawn launch of the Boeing Starliner, a space capsule the company built for NASA.