Four bolts used to secure the panel that ultimately blew off an Alaska Airlines plane during a flight last month were removed — and appear not to have been replaced — at Boeing’s factory in Renton, Wash., according to a preliminary report released Tuesday by the National Transportation Safety Board.

The panel, known as a door plug, was opened to repair damaged rivets on the plane’s body, known as the fuselage. The report did not say who removed the bolts keeping the door plug in place. But the safety board said it appeared that not all the bolts were put back once the door was reinstalled on the plane after the rivets had been repaired.

As evidence, the N.T.S.B. provided a photograph of the door plug after it was reinstalled but before the plane’s interior was restored. In the image, three of the four bolts appear to be missing. The location of the fourth bolt is covered with insulation.

The report said the image had been attached to “a text message between Boeing team members on September 19, 2023.” The Boeing employees “were discussing interior restoration after the rivet rework was completed during second shift operations that day,” the report said.

The safety board said there was no evidence that the plug was opened again after it left Boeing’s factory. The plane was delivered to Alaska Airlines at the end of October.