The initial findings into the incident, which saw a door blow off the Alaskan Airlines plane shortly after takeoff in January, have been released by the U.S. National Transportation Safety Board.
The report found that four key bolts, which lock the unused door to the fuselage, appeared to be missing.
Boeing has taken accountability for what happened, the report said.
"An event like this must not happen on an airplane that leaves our factory. We simply must do better for our customers and their passengers,” Boeing’s president Dave Calhoun said in a statement.
"We are implementing a comprehensive plan to strengthen quality and the confidence of our stakeholders.”
A piece of fuselage tore off the left side of the jet as it climbed following take-off from Portland, Oregon, en route to Ontario, California, forcing pilots to turn back and land safely with all 171 passengers and six crew on board.
A section of the fuselage reserved for the optional door had vanished, leaving a neat door-shaped gap.
The extra door is typically installed by low-cost airlines using extra seats that require more paths for evacuation.
According to the report, the missing bolts meant this panel was able to move out of position, and breakaway from the aircraft.
With AAP.
