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

Boeing said it would cooperate with the investigation and one being conducted by the National Transportation Safety Board.

The 63-pound door plug that blew off an Alaska Airlines Max 9 was found near Portland, Oregon, and will be be examined in the NTSB's 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.

"After the incident, the FAA was notified of additional discrepancies on other Boeing 737-9 airplanes," an FAA official wrote. Alaska and United Airlines reported finding loose bolts on door plugs that they inspected in some of their other Max 9 jets.

The FAA asked Boeing to respond within 10 business days and tell the agency "the root cause" of the problem with the door plug and steps the company is taking to prevent a recurrence.

"We will cooperate fully and transparently with the FAA and the NTSB on their investigations," said Boeing, which is headquartered in Arlington, Virginia.

Earlier this week, Boeing CEO David Calhoun called the incident "a quality escape." He told employees that the company was "acknowledging our mistake ... and that this event can never happen again."

The door plugs are installed by Boeing supplier Spirit AeroSystems, but investigators have not said which company's employees last worked on the plug on the Alaska plane that suffered the blowout.

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.

NTSB investigators 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.

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