Federal officials are investigating whether Boeing failed to make sure a panel that blew off a jetliner in midflight last week was safe and manufactured to meet the design that regulators approved.

The Federal Aviation Administration said Thursday that the investigation is focusing on plugs used to fill spots for extra exits when those doors are not required on Boeing 737 Max 9 jetliners.

The plug that blew off an Alaska Airlines Max 9 was found near Portland, Oregon, and will be examined in a government laboratory.

“This incident should have never happened and it cannot happen again,” the FAA said in a statement. “Boeing’s manufacturing practices need to comply with the high safety standards they’re legally accountable to meet.”

The FAA notified Boeing of the investigation in a letter dated Wednesday and asked it to respond within 10 business days.

The agency said Boeing’s response should include “the root cause of the encountered condition(s)” and steps the company is taking to prevent a recurrence.

The FAA has grounded Max 9 jets, including all 65 operated by Alaska and 79 used by United Airlines until Boeing can develop inspection guidelines and planes can be examined. Alaska has canceled all flights by Max 9s through Saturday.

Investigators with the National Transportation Safety Board said this week they have not been able to find four bolts that are used to help secure the 63-pound door plug. They are not sure whether the bolts were there before the plane took off.

Despite a hole in the side of the plane, pilots were able to return to Portland and make an emergency landing. No serious injuries were reported.