Boeing officials “inappropriately coached” test pilots during efforts to recertify 737 Max aircraft after they were grounded in March 2019 following two fatal crashes that killed 346 people, US Senate investigators said in a report. A simulator test was conducted for over a year by the Federal Aviation Administration (FAA) as a part of the recertification process to ensure that the aircraft could fly safely.

According to the report, the staff of the US Senate Committee on Commerce, Science, and Transportation started receiving information from whistleblowers after the second 737 Max crash, detailing numerous concerns related to aviation safety. The investigators said that the scope and breadth of the probe quickly expanded beyond the first allegations and dozens of whistleblowers revealed common themes.

Here are some of the most significant findings:

1. The report suggests that the FAA continues to retaliate against whistleblowers instead of welcoming their disclosures in the interest of aviation safety.

2. The investigators accused Boeing and the FAA of “attempting to cover up important information” that may have been contributed to those fatal crashes.

3. The US agency repeatedly permitted Southwest Airlines to continue operating dozens of aircraft in an unknown airworthiness condition for several years, which put millions of passengers at potential risk.

4. Boeing inappropriately influenced the FAA’s human factor simulator testing of pilot reaction times involving a Maneuvering Characteristics Augmentation System (MCAS) failure.

5. The senior officials at the FAA may have obstructed a review of 737 Max crashes conducted by the Department of Transportation Office of Inspector General.

Last month, Boeing Co received the approval of the FAA, which was required to resume its 737 Max flights, however, the approval didn’t allow the Max aircraft to immediately return to the skies. The FAA had rescinded the order that grounded the aircraft and published an Airworthiness Directive, which specifies design changes that must be made before the aircraft returns to service. The US regulator also detailed training changes that the aviation company must incorporate to resume commercial flights after a 20-month grounding. On December 9, commercial flights with Boeing 737 Max jetliners resumed for the first time since they were grounded worldwide.