A new report from the US National Transportation Safety Board has revealed concerning findings about a Boeing 737 Max incident that occurred in January. The report suggests that a door on an Alaska Airlines plane may not have been properly secured, as four key bolts meant to lock the unused door to the fuselage appeared to be missing.

The incident occurred shortly after take-off, when a panel covering an unused emergency exit, known as a door plug, suddenly blew out, leaving a hole in the aircraft’s fuselage. The missing bolts allowed the door panel to move out of position and break away from the aircraft, resulting in a rapid loss of cabin pressure.

The door plug, manufactured by Boeing’s supplier Spirit AeroSystems, was initially installed in the fuselage before being delivered to Boeing. However, it was later removed in the factory due to damage during the production process. Photographic evidence suggests that when the plug was reinstalled, at least three of the four locking bolts were not put back in place.

The report’s findings are expected to raise concerns for Boeing, which has already faced criticism over its corporate culture and quality control processes. Inspections have revealed loose bolts and fixings on other planes of the same specification, prompting questions about the manufacturing processes.

Prior to the Alaska Airlines incident, the 737 Max production line had experienced other serious problems, including manufacturing defects affecting key parts of the planes and a part protecting the central fuel tank against lightning strikes. The scrutiny is heightened due to the history of the 737 Max itself, as the new version of Boeing’s longstanding workhorse has faced significant challenges.