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 with a hole in the side of the plane.

Without the bolts, nothing prevented the panel from sliding upward and detaching from "stop pads" that secured it to the airframe.

The preliminary report said the door plug, installed by supplier Spirit AeroSystems, arrived at Boeing's factory near Seattle with five damaged rivets around the plug. A Boeing crew replaced the damaged rivets, which required them to remove the four bolts to open the plug.

The Federal Aviation Administration is separately investigating whether Boeing and its suppliers followed proper safety procedures in manufacturing parts for the Max. The FAA has barred Boeing from speeding up production of 737s until the agency is satisfied about quality issues.

FAA Administrator Michael Whitaker said Tuesday that his agency is about halfway through a six-week audit of manufacturing processes at Boeing and its key supplier on the Max, Spirit AeroSystems. He said the agency is confronted with two questions—what's wrong with the Max 9, and "what's going on with the production at Boeing?"

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