A door panel that blew off of an Alaska Airlines plane while it was flying over Oregon last month appears to have been missing four bolts, according to a preliminary report released Tuesday by the National Transportation Safety Board.

The bolts that secure the panel to the fuselage had been removed from the Boeing 737 Max 9 during repair work at Boeings factory in Renton, Washington, and apparently were not replaced, the report says.

Photos taken during the repair work in September show no evidence that the bolts were replaced before the panel, called a door plug, was closed by workers, the report states.