The Federal Aviation Administration on Saturday ordered U.S. airlines to stop using some Boeing 737 Max 9 planes until they were inspected, less than a day after one of those planes lost a chunk of its body in midair, terrifying passengers until the plane landed safely.

Alaska and United Airlines on Saturday began canceling dozens of flights after grounding their Max 9 fleets so the planes could undergo the federally mandated inspections.

Alaska Airlines Flight 1282 took off from Portland, Ore., on Friday, bound for Ontario, Calif., but was diverted back to Portland six minutes later, according to FlightAware, a flight tracking website. Those on board the flight described an unnerving experience, with wind blowing through a gaping hole that showed the night sky and the city lights below. The plane landed about 20 minutes after it had taken off, and no one aboard was seriously injured.

A passenger, Vi Nguyen of Portland, said that she woke up to a loud sound during the flight. “I open up my eyes and the first thing I see is the oxygen mask right in front of me,” Ms. Nguyen, 22, said. “And I look to the left and the wall on the side of the plane is gone.”

“The first thing I thought was, ‘I’m going to die,’” she added.

The National Transportation Safety Board sent a team to Portland to begin its investigation into the incident.

While the F.A.A. has yet to publicly discuss what caused the incident, it ordered airlines to inspect what it called a “mid cabin door plug.” Some of the Boeing 737 Max 9s are configured with fewer seats and, therefore, do not need all the exits originally designed for the plane. The unneeded doors are filled with a plug. The Alaska Air plane had two of those unneeded doors, located between the rear of the plane and the wing emergency exits, that were “plugged.”