The National Transportation Safety Board issued a preliminary report into last month's Alaska Airlines door plug blowout, saying evidence shows that four bolts were missing from the apparatus at the time of the incident.

In the wake of the incident, 171 Boeing 737 Max 9 aircraft, which had the same mid-cabin door plug, as the plane involved in the Alaska Airlines incident, were grounded, and Boeing has been under intense scrutiny from regulators since the explosive decompression on Jan. 5. The plug is referred to as an MED plug in the agency's report.
According to the NTSB, "two vertical movement arrestor bolts, two upper guide track bolts, forward lower hinge guide fitting, and forward lift assist spring were missing and have not been recovered."

The agency reports a lack of "damage patterns and absence of contact damage or deformation around holes" where the bolts should have been fastened. The "recovered aft lower hinge guide fitting indicate that the four bolts that prevent upward movement of the MED plug were missing before the MED plug moved upward off the stop pads," the report says.