Associated Press

Bolts that helped secure a panel to the frame of a Boeing 737 Max 9 were missing before the panel blew off the Alaska Airlines plane last month, according to accident investigators.

The National Transportation Safety Board issued a preliminary report on the Jan. 5 incident Tuesday.

The report included a photo from Boeing, which worked on the panel, which is called a door plug. In the photo, three of the four bolts that prevent the panel from moving upward are missing. The location of the fourth bolt is obscured.

The investigators said that the lack of certain damage around the panel indicates that all four bolts were missing before the plane took off from Portland, Oregon.

Pilots were forced to make a harrowing emergency landing.