WASHINGTON — After a six-week audit of Boeing, federal regulators say they found quality control problems at Boeing and Spirit AeroSystems, one of its top suppliers.

The Federal Aviation Administration says it found "multiple instances" of Boeing and Spirit failing to "comply with manufacturing quality control requirements."

The FAA launched the audit of Boeing and Spirit AeroSystems, which builds the fuselage for the Boeing 737 Max, after a door plug panel blew out in midair during an Alaska Airlines flight on January 5.

No one was seriously hurt when the plug came off as the new jet climbed through 14,000 feet after departing Portland, Ore. It returned to make an emergency landing as winds whipped through a hole in the fuselage.

A preliminary investigation by the National Transportation Safety Board determined four key bolts that were supposed to hold the door plug in place were missing when the plane left Boeing's factory.

The audit found problems in "Boeing's manufacturing process control, parts handling and storage, and product control," the FAA said in a statement.

The agency says FAA administrator Mike Whitaker discussed the findings with Boeing CEO Dave Calhoun last week, when the agency gave Boeing 90 days to come up with a plan of action to address its quality control problems.

The FAA says it provided both companies with a summary of the audit findings. But the agency declined to share those details with NPR, citing its ongoing investigation.

Auditors visited Boeing's factory in Renton, Wash. and Spirit's plant in Wichita, Kan.

Boeing confirmed Friday that it is in talks to buy Spirit.

"We believe that the reintegration of Boeing and Spirit AeroSystems' manufacturing operations would further strengthen aviation safety, improve quality and serve the interests of our customers, employees, and shareholders," said Jessica Kowal, Boeing's director of media relations, in a statement.

That would be a change of strategy for Boeing, which nearly two decades ago sold off the assets that are now part of Spirit.

But the supplier has had several costly and embarrassing problems with quality control in recent years as it pushed to keep up with Boeing's ambitious production schedule.

NPR's Joel Rose reported from Washington, D.C. and Russell Lewis from Birmingham, Ala.