Report: United Airlines, Alaska Airlines Find 'Loose Bolts' on Plane Doors Following Inspections Preliminary inspections on grounded Boeing 797 Max 9 aircraft began after a door flew off an Alaska Airlines plane mid-flight on Friday.

United Airlines and Alaska Airlines said that preliminary investigations of Boeing 737 Max 9 aircraft in their respective fleets have found loose hardware and bolts on some door plugs.

"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," United Airlines explained in a statement, per ABC News. "These findings will be remedied by our Tech Ops team to safely return the aircraft to service."

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The news comes after a plane door panel blew out mid-flight on an Alaska Airlines aircraft on Friday. The Federal Aviation Agency grounded over 170 Boeing 737 Max 9 planes for emergency inspections following the incident, causing hundreds of cancelations across both airlines.

United did not specify how many of its 737 Max 9 aircraft were found to have loose hardware. The airline currently operates 79 of the models.

Alaska Airlines made a similar statement, noting that none of the aircraft would fly again until formal inspections and investigations were completed.

"No aircraft will be returned to service until all of these steps are complete," Alaska Airlines wrote in a release regarding its 65 Boeing 737 Max 9 aircraft. "The safety of these aircraft is our priority and we will take the time and steps necessary to ensure their airworthiness, in close partnership with the FAA. "

Boeing, whose valuation is down over 9% in the past five days following the news of Friday's accident, released a public statement late Monday: "As operators conduct the required inspections, we are staying in close contact with them and will help address any and all findings. We are committed to ensuring every Boeing airplane meets design specifications and the highest safety and quality standards. We regret the impact this has had on our customers and their passengers."

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No passengers were injured during Friday's incident, and no one was seated in the two passenger seats next to the door.

As of late Tuesday morning, United had canceled another 203 flights and Alaska Airlines had canceled 108.