A Boeing 737 MAX 9 taxis following a landing during a first-flight event for the company's new airplane at Boeing Field in Seattle, Washington on April 13, 2019. (JASON REDMOND/ AFP)

The families of victims killed in two crashes by Boeing Co’s 737 MAX want US regulators to rescind approval for the planes to fly again, following a Senate report that raised concerns about the re-approval process.

The Federal Aviation Administration (FAA) lifted its 20-month safety ban of the 737 MAX on Nov 18 after green-lighting design changes by Boeing to address issues involved in two fatal crashes in Indonesia in late 2018 and in Ethiopia in early 2019.

A lengthy Senate Commerce Committee report released on Friday found that Boeing officials “inappropriately coached” FAA test pilots during its recertification efforts

But a lengthy Senate Commerce Committee report released on Friday found that Boeing officials “inappropriately coached” FAA test pilots during its recertification efforts.

“The Committee found wrongful and potentially unlawful behavior that must be investigated to determine whether the MAX recertification process was tainted,” families of Ethiopian crash victims said in the Dec 22 letter to the FAA and Department of Transportation that was reviewed by Reuters.

READ MORE: US Senate probe finds Boeing, FAA manipulated 737 Max tests

Asked for comment, the FAA said it had worked closely with other international regulators to conduct “a thorough and deliberate review” of the 737 MAX.

“We are confident that the safety issues that played a role in the tragic accidents involving Lion Air Flight 610 and Ethiopian Airlines Flight 302 have been addressed through the design changes required and independently approved by the FAA and its partners,” a spokesman said.

Boeing declined to comment. When the Senate report was released on Dec 18, Boeing said it takes “seriously the committee’s findings and will continue to review the report in full.”

“With the first US flight of the 737 MAX set to occur within days, we call for the plane to be immediately grounded based upon the new revelations,” the families wrote.

A total of 346 people died in the two crashes.

ALSO READ: Boeing's updated 737 completes 1st flight with media onboard

American Airlines is set to fly the plane on Dec 29 for the first time in the United States with commercial passengers since the plane’s global grounding in March 2019.