US Senate investigators say that Boeing officials "inappropriately coached" test pilots during efforts to recertify the company's 737 Max aircraft.
The planes were grounded in March 2019 following two deadly crashes.
Investigators accused Boeing and Federal Aviation Administration (FAA) officials of "attempting to cover up important information".
Boeing said it was reviewing the findings and took them "seriously", while the FAA defended its conduct.
The FAA said the Senate Commerce Committee's report contained "a number of unsubstantiated allegations", and that its review of the 737 Max had been thorough. It said it was confident that safety issues with the aircraft had been addressed.
The crashes in Indonesia and Ethiopia came within five months of each other and together killed 346 people. They have been attributed to flaws in automated flight software called MCAS, which prompted the planes to nosedive shortly after take-off.