It seems bolts were not installed on the Alaska Airlines flight

ALASKA BOEING 737-890 N548AS (MSN 30020)

The 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 fuselage, according to Boeing’s records. 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 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 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.

Until now, the NTSB had not said what actually caused the panel to rip off the Alaska Airlines-operated jet as the plane climbed to 16,000 feet on the 5th of January.

The US Federal Aviation Administration grounded all 171 Boeing 737 MAX 9 planes after the incident, most operated by US carriers United and Alaska Airlines, for inspections.

Those planes were cleared to return to service in late January.

The NTSB has been focused on how the panel — fitted into this MAX 9 model in place of an optional exit — detached from the plane. The plug is held down by four bolts and then secured by "stop fittings" at 12 different locations along the side of the plug and the door frame.

All 12 stop fittings became disengaged during the event, the NTSB said in January.

Both United Airlines and Alaska Air said in the days after the blowout they had found loose parts on multiple grounded MAX 9 aircraft.

The plug was manufactured by Spirit Aero Systems the one-time subsidiary of Boeing which separated from its parent in 2005.