The Federal Aviation Administration said Monday its audit of 737 Max manufacturing at airplane-maker Boeing and its key supplier turned up “multiple instances” of them failing to make sure manufacturing met quality standards.

The FAA said that it found “non-compliance issues” with Boeing's manufacturing-process control and parts handling and storage. It did not provide details.

The FAA said it gave a summary of findings from its six-week audit to Boeing and supplier Spirit AeroSystems, but it did not make the summary public. A spokeswoman said the FAA can't release details because its investigation of Boeing is continuing.

Spirit spokesman Joe Buccino said the company welcomed the FAA audit and will review the findings. “We are in communication with Boeing and the FAA on appropriate corrective actions,” he said.

Asked for comment, a Boeing spokesman referred to a statement last week in which CEO David Calhoun said the company now has a “clear picture of what needs to be done” and is "totally committed to meeting this challenge.”

The FAA has stepped up its scrutiny of Boeing since Jan. 5, when a panel blew off a Boeing 737 Max 9 as it flew 16,000 feet above Oregon. Pilots of the Alaska Airlines jet were able to safely land the plane despite the hole in its side.

Since then, Boeing replaced the executive in charge of the 737 Max program. Last week, the FAA gave Boeing 90 days to come up with a plan for addressing safety concerns raised by the FAA and an independent panel of experts from industry, government and academia.