Updated Jan. 9, 2024 9:55 am ET

United Airlines and Alaska Airlines have discovered loose parts on Boeing 737 MAX 9 jets that they have inspected after a near-catastrophe on a flight Friday, signaling Boeing’s issues go beyond the aircraft that made an emergency landing.

The disclosures came shortly before investigators at the National Transportation Safety Board said that dangerous episode on an Alaska Airlines jet occurred because an emergency exit-sized door plug blew out at around 16,000 feet after somehow moving off a set of stops that are designed to keep it attached to the plane.

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