New York, New York - More than 300 flights were canceled across the United States on Monday after 171 Boeing 737 Max 9 planes were grounded following an accident on Alaska Airlines.

The aircraft were shut down for inspections after a piece of the fuselage on one blew out during an Alaska Airlines flight from Portland, Oregon, Friday night.

The decision to ground the planes affected Alaska and United Airlines the most, as both carriers usually fly dozens of 737 Max 9 planes.

Boeing and the Federal Aviation Administration sent inspection instructions to all airlines on Monday morning about the 737 Max 9, according to a statement from Alaska Airlines.

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"Our technicians have prepared each aircraft to be immediately ready for the required inspection when instructions are finalized," the company said in its statement.

Alaska Airlines Flight 1282 was about 16,000 feet in the air en route to Ontario, California, from Portland when an emergency exit door plug detached from the plane, leaving a massive hole in the main cabin.

The aircraft quickly turned around and landed back in Portland about 35 minutes after its initial departure. No one was killed or seriously injured in the terrifying incident.

On Sunday night, the blown-out door plug was located in a teacher's backyard in Portland. National Transportation Safety Board chair Jennifer Homendy said it would be a crucial piece of evidence in the investigation into the crash.

Alaska Airlines operates 65 Boeing 737 Max 9 aircraft. The carrier canceled 144 flights on Monday, according to the flight tracking website FlightAware. In a statement, Alaska said "roughly 140" cancellations were caused by the 737 Max 9 grounding.

United Airlines usually flies 79 Boeing 737 Max 9s. According to FlightAware, 229 United flights were canceled on Monday.