An Alaska Airlines flight made an emergency landing at Portland International Airport in Oregon on Friday evening after experiencing what the federal authorities described as a midair pressure problem that passengers said blew out a chunk of the fuselage.

The airline said that Alaska Airlines Flight 1282 had made a safe emergency landing carrying 171 passengers and six crew members after returning to the Portland airport shortly after takeoff for Ontario, Calif. The crew reported a “pressurization issue” before landing, the Federal Aviation Administration said in a separate statement.

A passenger, Vi Nguyen of Portland, said that she woke up to a loud sound during the flight. Then she saw a large hole in the side of the aircraft.

“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.

Her friend, Elizabeth Le, 20, said she had also heard “an extremely loud pop.” When she looked up, she saw a gaping hole on the wall of the plane about two or three rows away, she said.

Ms. Le said no one was sitting in the window seat next to the missing fuselage but that a teenage boy and his mother were sitting in the middle and aisle seats. Flight attendants helped them move to the other side of the plane a few minutes later, she said. The boy appeared to have lost his shirt somehow, and his skin looked red and irritated, she added.

“It was honestly horrifying, I almost broke down, but I realized I needed to remain calm,” she said.

There were announcements over the speaker system but none were audible because the wind whipping through the plane was so loud, she said. After the plane landed, paramedics came on board to ask whether anyone was injured, she added. A man seated in the row immediately behind the hole said that he had hurt his foot.

Ms. Le said the passengers were not given an explanation of what had happened. In a video she took of the flight, passengers can be heard clapping after landing. “Oh, my god,” someone says.

The plane was a Boeing 737 Max 9, according to FlightAware, a flight tracking website. The airline, the Federal Aviation Administration and the National Transportation Safety Board said that they were investigating what had happened.

Ms. Nguyen, who was traveling with her friends, said that after landing they had been told they could board another flight to Ontario later that night.

Alaska Airlines Flight 1282 departed for Ontario International Airport at 5:07 p.m., according to FlightAware, and was diverted back to Portland six minutes later. It reached a maximum altitude of about 16,000 feet, when its speed was recorded at more than 440 miles per hour, and landed in Portland at 5:27 p.m.

Boeing said in a statement that it was “aware of the incident involving Alaska Airlines Flight 1282,” adding: “We are working to gather more information and are in contact with our airline customer.”

The 737 Max has been scrutinized by the authorities in recent years. In December, Boeing urged airlines to inspect all 737 Max airplanes for a possible loose bolt in the rudder-control system after an international airline discovered a bolt with a missing nut during routine maintenance. Alaska Airlines said at the time that it expected to complete inspections for its fleet in the first half of January.

That was another development in what has been a troubled history for the plane, a single-aisle workhorse aircraft that was designed for short and intermediate distances.

In 2018, Lion Air Flight 610 crashed into the ocean off the coast of Indonesia, killing all 189 passengers and crew members. Less than five months later in 2019, Ethiopian Airlines Flight 302 crashed shortly after leaving Ethiopia’s capital, Addis Ababa, killing all 157 people on board.

The two crashes prompted regulators around the world to ground the Max. Boeing made changes to the plane, including to the flight control system behind the crashes, and the Federal Aviation Administration cleared it to fly again in late 2020. In 2021, the company agreed to a $2.5 billion settlement with the Justice Department, resolving a criminal charge that Boeing conspired to defraud the F.A.A.

Mark Walker contributed reporting.