Boeing officials "inappropriately coached" test pilots during recertification efforts after two fatal 737 MAX crashes killed 346 people, according to a lengthy congressional report released on Friday.

The report from the Senate Commerce Committee Republican staff said testing this year of a key safety system known as MCAS tied to both fatal crashes was contrary to proper protocol.

The committee concluded Federal Aviation Administration (FAA) and Boeing officials "had established a pre-determined outcome to reaffirm a long-held human factor assumption related to pilot reaction time ... It appears, in this instance, FAA and Boeing were attempting to cover up important information that may have contributed to the 737 MAX tragedies."

The report citing a whistleblower who alleged Boeing officials encouraged test pilots to "remember, get right on that pickle switch" prior to the exercise that resulted in pilot reaction in approximately four seconds, while another pilot in a separate test reacted in approximately 16 seconds.

The account was corroborated during an FAA staff interview, the committee added.

Numerous reports have found Boeing failed to adequately consider how pilots respond to cockpit emergencies in its development of the 737 MAX.

Boeing said Friday it takes "seriously the committee’s findings and will continue to review the report in full."

Senate Commerce Committee chairman Roger Wicker said the report "details a number of significant examples of lapses in aviation safety oversight and failed leadership in the FAA."

The committee also said "multiple independent whistleblowers contacted the committee to allege FAA senior management was complicit in determining the 737 MAX training certification level prior to any evaluation."

Boeing (BA) resisted requiring simulator training for pilots before operating the 737 MAX but reversed course in January.

The report also noted Southwest Airlines was able to operate more than 150,000 flights carrying 17.2 million passengers on jets without confirmation that required maintenance had been completed.

The Senate report said the Southwest flights "put millions of passengers at potential risk." Southwest did not immediately comment.

Boeing still faces an ongoing criminal probe into the MAX. The committee said its review was "constrained due to the continued criminal investigation"

Last month, the FAA approved the 737 MAX's return to service and flights have resumed in Brazil. The first U.S. 737 MAX commercial flight with paying passengers is set for Dec. 29.

Last month, the Senate committee unanimously a bill to reform how FAA certifies new airplanes and grant new protections for whistleblowers, among other reforms, while the U.S. House of Representatives unanimously passed a similar bill.