Four key bolts were missing from the door that blew off an Alaska Airlines plane in mid-air, according to a preliminary report from a US regulator.
The door plug had been removed to fix rivets damaged in production, the initial report said.
The investigation was commenced by the investigative agency National Transportation Safety Board (NTSB) in early January after a mid-flight blowout where the door flew off a Boeing 737 MAX 9 Alaska Airlines aircraft.
The door plug is held down by four bolts and secured by stop fittings at 12 different locations along the side of the plug and door frame.
The NTSB said in January that all 12 stop fittings disengaged during the flight.
The Federal Aviation Authority (FAA) temporarily grounded all 171 of the MAX 9 jets and placed an unprecedented ban on raising production volumes within the entire 737 MAX fleet of aircraft.
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Thousands of flights were cancelled by Alaska Airlines and United Airlines, users of the Boeing 737 MAX 9, as a result of the grounding order, which was lifted on 24 January.
The FAA said 94% of the jets have returned to service.