Four key bolts were missing from the door that blew off an Alaska Airlines plane in mid-air in January, according to a preliminary report from a US regulator.
The door panel, known as a plug, had been removed to fix rivets damaged in production, the initial report said.
Those bolts were removed at a Boeing factory in Washington and appear not to have been replaced, it said.
Text messages sent in September between employees at the plane maker show a discussion about interior restoration after the rivet rework. A photo attached to one of the text messages formed part of the probe.
There was no evidence the door was opened again after it left the Boeing factory, the report added, increasing pressure on one of the world's two biggest plane makers.
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 investigation continues to determine what manufacturing documents were used to authorize the opening and closing of the plug during the rivet rework," the report said.
Records from September showed five rivets were damaged on the frame of the door plug.
Boeing president and chief executive Dave Calhoun said on Tuesday: "Whatever final conclusions are reached, Boeing is accountable for what happened. An event like this must not happen on an airplane that leaves our factory."
The manufacturer added it "implemented a control plan to ensure all 737-9 mid-exit door plugs are installed according to specifications."
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.
Both Alaska Air and United Airlines said in the days after the blowout they had found loose parts on multiple grounded MAX 9 aircraft.
The FAA said 94% of the jets have returned to service.
The regulator indefinitely limited Boeing's ambitious MAX plans production, raising questions about the company's manufacturing future.
The current safety system isn't working, FAA head Mike Whitaker told the US politicians of the Committee on Transportation and Infrastructure on Tuesday.
"I certainly agree that the current system is not working, because it's not delivering safe aircraft."