Four days after a section of the fuselage suddenly detached as an Alaska Airlines plane was taking off, United Airlines reported Monday its inspectors discovered several “instances that appear to relate to installation issues in the door plug,” including bolts needing additional tightening.

United Airlines said it has been inspecting its fleet of Boeing 737 Max 9 planes to make sure they are safe to fly as federal investigators continued to investigate Friday’s near-disaster aboard an Alaska Airlines plane.

The announcement from United came a day after National Transportation Safety Board Chair Jennifer Homendy reported they had found a missing door plug that could be key to the investigation of the midair accident in the backyard of a home of a Portland, Oregon, teacher identified only as “Bob.”