The pilots were forced to make an emergency landing in Portland, Oregon, but no one was injured in the incident.
The evidence is based in part on the aircraft manufacturer’s photos of the panel that fell from the plane. Lack of damage around the bolt holes leads the NTSB to conclude that all four bolts were missing.
The panel was a kind of plug that replaced an emergency exit on the Boeing 737 Max 9 model aircraft. American authorities issued a flight ban for the model over the American continent until the plane was checked.