(NewsNation) — After a door plug blew off an Alaska Airlines plane midflight last week, federal investigators are looking into the bolts that were supposed to hold the panel in place.

Ahead of the formal federal investigation, both Alaska Airlines and United Airlines have already found loose hardware around door plugs on some of the grounded planes.

The National Transporation Safety Board is aware of their findings and is collecting other data, including the three previous incidents of pressurization warnings on the place in question.

Meanwhile, the NTSB investigation has indicated other potential failures may have been averted by grounding the Boeing Max 9s.

Investigators are wrapping up their work on the Portland site after poring over the jet involved and packing up the retrieved pieces, including the 63-pound door plug.

According to the NTSB, the bolts that should hold the door plug in place are missing. They are now looking into whether the bolts were missing when the plane took off.

“It is the two guides on the top that broke. Those have two bolts in them. We don’t know if they were there or if they are just missing and departed when the door plug departed,” said NTSB chair Jennifer Homendy.

Vivki Kreps was flying with her two grandchildren when the door plug blew out seven rows behind her.

“A big gust of wind hit us in the face, and we were pushed back into our chairs. The U.S. oxygen masks fall. Eventually, there was a mist in the air that I noticed. I thought at the time it was smoke and so I lowered my mask slightly to be able to smell and kind of perked up and looked around and over my left shoulder and I could see there was a big hole in the plane,” Kreps told NewsNation.

The incident led to the grounding of over 170 Boeing 737 Max 9s as authorities launched an investigation into what exactly happened. A key piece of the investigation, the missing door plug that flew off, was found by a Portland school teacher.

Bob Sauer discovered the door plug in his backyard a few days after the incident.

“I could see that there was something gleaming white underneath the trees in the back that isn’t normally there. And when I went to investigate it, it was very obviously part of a plane. My heart did start beating a little faster at that point because I thought, ‘Oh my goodness, people have been looking for this all weekend, and it looks like it’s in my backyard,'” Sauer said.

Boeing manufacturers often order different seating configurations as fewer passengers allow for fewer emergency exits, so door plugs are used to fill those.

The NTSB investigation will focus on Spirit AeroSystems, which produces the body of the 737 Max 9s for Boeing. They plan to do 3D scans of both the door plug and the fuselage opening to match them to models and drawings.