Investigators have begun focusing on a set of bolts used to fasten the door plug in place on Boeing 737 Max 9 aircraft as a possible cause of the fuselage failure last week on an Alaska Airlines flight shortly after takeoff from Portland International Airport.

Officials with the National Transportation Safety Board (NTSB) have confirmed the four retaining bolts were not attached to either the door or the plane following the harrowing midair incident on which The Dallas Express reported.

More than 170 Boeing 737 Max 9 aircraft remain grounded as the investigation continues, causing hundreds of flight cancellations by Alaska and United Airlines, the two commercial airlines that use the planes.

Federal Aviation Administration inspectors have begun to focus on Spirit AeroSystems, a spin-off company responsible for manufacturing fuselage parts, WFAA reported. The company was also at the root of two fatal airline crashes involving Boeing 737-8 aircraft that triggered a U.S. House investigation.

Inspections of other aircraft have revealed loose and missing bolts in various parts of the fuselage, indicating a potential widespread quality control issue. Ten aircraft have now been identified with compromised parts.

The issue only applies to 737 Max 9 aircraft that use door plugs to replace emergency exit doors in planes with lower seating capacity. The plug is secured with four bolts and 12 connecting points. Investigators know that the plug moved upward before blowing off the Alaska flight but have not confirmed the reason for the failure.

The door plug was recovered from a residential yard outside Portland, along with some passengers’ belongings. No one was injured in the failure, but one teenage boy had his shirt ripped off as the cabin depressurized. The plane made an emergency landing in Portland after the incident.

The extended grounding comes as investigators continue to inspect aircraft in the fleet. Inspections are happening at a pace slower than United and Alaska had hoped, per WFAA. The FAA has ensured the grounding will continue until the administration is confident problems with the aircraft have been corrected.

A federal securities lawsuit filed in May 2023 claims widespread manufacturing problems at Spirit, including managers that encourage inspectors to cover up issues that could lead to significant problems, such as the failure of the door plug, WFAA reported.