The door that blew off a Boeing 737 Max airplane during an Alaska Airlines flight appeared to be missing bolts that are supposed to keep the panel secured ... per a new report.
In a preliminary report from its investigation, the NTSB says four bolts meant to lock the door to the body of the plane were removed by Boeing to fix another issue and appear not to have been replaced.
The NTSB says the bolts were removed at a Boeing factory when the door was opened as part of a repair on damaged rivets on the plane's fuselage.
It's unclear who removed the bolts, the NTSB report doesn't say, though the safety board says it looks like at least 3 and possibly all 4 bolts were never reinstalled after the rivet repair.
The NTSB says Boeing removed the bolts for a September repair and there's no evidence the door was ever opened again before the 737 Max was delivered in October to Alaska Airlines.
As we reported ... the door blew off back in January, shortly after Alaska Airlines Flight 1282 took off from Portland International Airport.
The plane, which was packed with passengers, had to make an emergency landing.
Boeing's 737 Max planes were grounded as airlines checked the bolts on the doors ... and the aircraft was only recently given the green light to get back up in the sky, despite some saying it was too early.
The NTSB says it's still investigating the incident.