(NewsNation) — The National Transportation Safety Board is combing through evidence after part of an Alaska Airlines plane blew out midflight. That missing door plug has now been found.

As the investigation into the incident continues, United Airlines said Monday it found loose bolts and other “installation issues” on a part of some Boeing 737 Max 9 jets that were inspected after the blowout Friday.

“Since we began preliminary inspections on Saturday, we have found instances that appear to relate to installation issues in the door plug – for example, bolts that needed additional tightening,” Chicago-based United said.

The door plug that blew off the plane shortly after takeoff was found in the backyard of a Portland, Oregon, school teacher identified only as “Bob.” The NTSB says this piece is crucial in determining why the incident occurred.

The right side door of the plane appears to be fine and will be useful for comparisons.

The investigation into the incident revealed that a pressurization warning light on that aircraft had gone off during three previous flights, with one being just the day before. Each time, maintenance crews tested and then reset the light.

Because of that warning light, Alaska Airlines had blocked the plane from being used for flights over water in case the plane had to make an emergency landing.

Jennifer Homendy, chair of the NTSB, cautioned the pressurization light might be unrelated to Friday’s incident.

One setback investigators faced was a lack of the cockpit audio recording. The NTSB says the circuit breaker for preserving the recording was not pulled before the system automatically reset, causing the crucial recording to be overwritten.

According to the NTSB, the jet was almost new and went into service two months ago.

As the investigation continues, the Federal Aviation Administration grounded all Max 9s operated by Alaska and United and some flown by foreign airlines.