The National Transportation Safety Board of the United States said on the 24th that Boeing's insufficient training and guidance of employees in the manufacturing process and the Federal Aviation Administration's poor supervision were the main reasons for the cabin door plug (embedded emergency door) falling off shortly after takeoff on its 737 MAX 9 passenger aircraft in 2024. According to the Associated Press, the National Transportation Safety Board found after a 17-month investigation that the four bolts that fixed the door plug were removed and not reinstalled during the assembly of the passenger aircraft.