It felt like an explosion. Then the plane rattled, the oxygen masks dropped, and the lights flickered. A white vapor whipped through the cabin. Some people yelled. Others were disoriented.

So began the harrowing minutes aboard Alaska Airlines Flight 1282, when a door-sized section near the rear of the plane blew off 10 minutes after it took off from Portland, Ore., on Friday night, leaving some passengers initially confused, and others utterly terrified.

“The first thing I thought was, ‘I’m going to die,’” said Vi Nguyen, 22, a passenger from Portland.

Nicholas Hoch, 33, was sitting in 12A, a window seat near the front of the Boeing 737 Max 9. He tried to stay calm. Still, he began typing out texts to his mother and girlfriend. There was something wrong on the plane, he told them, adding, “I love you guys.”

The flight, carrying 171 passengers and six crew members, began in ordinary fashion. Headed for Ontario, Calif., it was initially delayed by about 20 minutes to allow for de-icing, said Mr. Hoch, an architect, who was flying to visit his girlfriend’s family.

He spent the delay getting into his preflight comfort zone, putting on his noise-canceling headphones and listening to a podcast about Tokyo, ahead of his trip to Japan in a couple of days.

At 5:07 p.m., Flight 1282 departed Portland, according to FlightAware, a flight tracking website. It climbed to about 16,000 feet, traveling roughly 440 miles per hour.

Then came the boom. To others, a bang. Vicki Kreps, 56, a nurse from Vancouver, Wash., who was sitting in Row 19 with her two grandchildren, Brady, 7, and Brynlee, 5, said they could feel a definite shift forward in their seats, then the decompression of air in their ears. She helped put Brynlee’s oxygen mask on, while Brady put his on himself.

“We definitely felt that we were descending quickly,” Ms. Kreps said.

Evan Smith, a 72-year-old lawyer, said he began to see “dusky, smoky stuff” swirling around the cabin.

Mr. Hoch looked up and saw passengers’ hair being blown back. “The best way I can describe it is like puncturing a CO2 canister and that vapor releasing out of the canister,” he said.

“But we were in that canister.”

A moment of hysteria broke out when a few people stood up, pointing and yelling that there was a hole in the back.

It was in Row 26, aviation authorities said in a news conference Saturday. No one was sitting in the window and middle seat. But after the “rapid decompression” of the cabin, the headrests of two nearby seats were gone, as well as the back of one.

Authorities who are investigating are still looking to locate the missing part, what they called a door plug, because it covers what is sometimes used in other models of the plane as an emergency exit. On Saturday, they said they believed the door plug was somewhere in a neighborhood outside of Portland, near Interstate 217.

After it blew off, a woman walked up near the front of the plane to say someone’s shirt had been ripped off, Mr. Hoch said. He also said people told him they lost possessions, like their phone or earbuds.

Flight attendants made announcements asking passengers to sit down and stay seated. But Elizabeth Le, 20, a friend of Ms. Nguyen, said they were difficult to hear because of the wind whipping through the aircraft.

Confusion spread throughout the cabin, with those sitting in the front not knowing what was happening behind them, Mr. Hoch said.

Ms. Le said a boy and his mother were sitting near the missing section. 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, and his skin looked red and irritated, she added.

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

Mr. Hoch said he felt a range of emotions. He was keeping his head on a swivel, constantly looking back and forth. The woman next to him was in tears. She asked him if he would hold her hand, he said, and he did.

But overall, everyone was relatively “eerily calm,” Mr. Hoch said, though he added that some “were silently freaking out.”

Mr. Smith, who was returning to his home in Murrieta, Calif., after visiting his daughter and son-in-law in Portland, said his previous experience as a military police officer taught him that it was important to keep a cool head. He had a feeling the plane was durable enough to land.

The flight circled back to Portland. “The plane was stable. It wasn’t shaking, it wasn’t making any weird maneuvers, it was just flying steady,” he said. “At that point, I was sure the aircraft was fine and we were going to get down OK.”

Ms. Kreps said that the flight crew helped people stay composed. “I’m super impressed with Alaska and how they handled this situation,” she said.

Passengers said that once the plane landed, at 5:27, paramedics came on board. A man seated in the row immediately behind the hole said that he had hurt his foot, but no major injuries were sustained.

In a video that Ms. Le took of the flight, passengers can be heard clapping after landing.

“Oh my God,” one says in the video.

John Yoon, Victoria Kim, Orlando Mayorquin, Niraj Chokshi, Mark Walker and Johnny Diaz contributed reporting. Susan Beachy contributed research.