In the wide-ranging investigation, Boeing failed a check which dealt with the component that blew off the jet, known as a door plug, the Monday report said, citing an FAA presentation viewed by NYT.

Boeing's shares were down 1.4% in premarket trading on Tuesday.

Supplier Spirit AeroSystems, which makes the fuselage for the MAX, passed six of 13 audits and failed the rest, the report added.

Additionally, an audit at Spirit focusing on the door plug component found five problems and it failed the one which dealt with the installation of the component, the report said.

The audit raised concerns about the technicians who carried out the work and found that the company "failed to determine the knowledge necessary for the operation of its processes," according to the report.

Other audits that Spirit failed included one that involved a cargo door and another that dealt with the installation of cockpit windows, it said.

Based on the FAA audit, Boeing is continuing to implement immediate changes, and is developing a plan to strengthen safety and quality, the plane maker told Reuters in an emailed statement.

The FAA and Spirit AeroSystems did not immediately reply to Reuters' requests for comment.

Earlier in the day, U.S. Transportation Secretary Pete Buttigieg said he expects Boeing to cooperate in investigations by the Justice Department and National Transportation Safety Board into the 737 MAX 9 mid-air emergency on Jan. 5.

Meanwhile, the FAA's Michael Whitaker said the agency and Boeing hope to define the milestones the manufacturer must meet in order to increase the MAX production rate in the next 30 days.

Last week, the agency said it found "non-compliance issues in Boeing's manufacturing process control, parts handling and storage, and product control."

• Boeing Whistleblower Dies From 'Self-Inflicted' Wound