An unidentified man who refused to be bumped from a plane screamed as a security officer wrestled him out of his seat and dragged him down the aisle by his arms. His glasses slid down his face, and his shirt rose above his midriff as uniformed officers followed.
At least two passengers documented the physical confrontation and the man’s anguished protests, and their videos spread rapidly online on Monday as people criticized the airline’s tactics. A security officer involved in the episode has been placed on leave, the authorities said, and the federal Transportation Department is investigating whether the airline complied with rules regarding overbooking.
Tyler Bridges, a passenger on Sunday’s flight who posted a video to Twitter, said in a telephone interview on Monday that “it felt like something the world needed to see.”
The shocking scene raised questions about the common practice of overbooking and how far airlines will go to sell all of their seats. Particularly annoying, Mr. Bridges said, was that the airline was looking for extra seats for some of its employees.
The videos show a security officer removing the unidentified man from his seat and dragging him off the plane as he screams. The flight was scheduled to depart O’Hare International Airport in Chicago for Louisville, Ky., at 5:40 p.m. but was delayed two hours.
Charlie Hobart, a United spokesman, said in a telephone interview on Monday that “we had asked several times, politely,” for the man to give up his seat before force was used.
“We had a customer who refused to leave the aircraft,” he said. “We have a number of customers on board that aircraft, and they want to get to their destination on time and safely, and we want to work to get them there.
“Since that customer refused to leave the aircraft, we had to call” the police, and they came on board, he said.
The Chicago Department of Aviation said in a statement on Monday that the incident “was not in accordance with our standard operating procedure” and that an officer had been placed on leave pending a review of the matter. The department declined to identify the officer.
Airlines routinely sell tickets to more people than the plane can seat, counting on several people not to arrive. When there are not enough no-shows, airlines first try to offer rewards to customers willing to reschedule their plans, usually in the form of travel vouchers, gift cards or cash. The arrangement — which is usually negotiated before passengers board the plane — can be lucrative to flexible travelers and is crucial for airlines to maximize profit.
“A bakery doesn’t want to have a lot of extra pastries at the end of the day they have to throw out,” said Seth Kaplan, managing partner at Airline Weekly, an industry publication. “To an airline, an empty seat is basically the same thing as stale bread. It’s something they can never sell again.”
But involuntarily bumping passengers is rare. In 2016, United involuntarily denied boarding to 3,765 people, or 0.00004 percent of its more than 86 million passengers on oversold flights, according to the Transportation Department. An additional 62,895 people voluntarily gave up their seats.
The event on Sunday was the second social media stir for United in two weeks. In March, two girls were barred from a flight because they were wearing leggings, which the company said violated its dress code for a benefit for United employees and their dependents. Critics called the policy sexist and overbearing.
On Sunday, Mr. Bridges said that when he arrived at the gate about 20 minutes before boarding, United had announced that the flight was overbooked; the airline was offering $400 vouchers to anyone who would give up their seat, Mr. Bridges said.
As the passengers boarded the plane, “there was no indication anything was wrong,” Mr. Bridges said.
An airline employee came on board and said United needed four people to get off, Mr. Bridges said, adding that the airline had by then increased its incentive to an $800 voucher. The airline later said that it offered up to $1,000 in compensation.
Mr. Hobart, the United spokesman, confirmed that United sought passengers willing to give up their seats with compensation but that none stepped forward.
Another United employee told passengers that the plane would not leave until four people got off, Mr. Bridges said. The employee specified that the airline had four United employees who needed to get to Louisville, he said.
Four passengers were selected to be bumped, and three left without incident, Mr. Hobart said.
Mr. Hobart would not say whether the bumped passengers were chosen by a computer, an employee or some combination of the two. But factors can include how long a customer would have to stay at the airport before being rebooked, he said, and the airline looks to avoid separating families or leaving unaccompanied minors.
A United employee first approached a couple who appeared to be in their mid-20s, Mr. Bridges said, and the pair begrudgingly got off the plane. Then the United employee went to a man five rows behind Mr. Bridges and told him he needed to get off the plane. Mr. Bridges said the man told the employee: “I’m not getting off the plane. I’m a doctor; I have to see patients in the morning.”
Mr. Hobart said: “We explained the scenario to the customer. That customer chose not to get out of his seat.”
The United employee then told the man that if he did not get off the plane, she would call security. As she turned to leave, the man shouted after her, Mr. Bridges said. Specifically, he said, the passenger complained that he had been singled out because he was Chinese.
“It was really intense, really uncomfortable,” he said.
The situation also became uncomfortable for the United employees who then got on board and took the vacated seats, Mr. Bridges said. They were berated by passengers and told they should be ashamed, he said.
The man who had been removed returned to the flight briefly, Mr. Bridges said. Video shows him jogging through the aisle, repeatedly saying, “I have to go home.”
Jayse Anspach, a seminary student who was also on the flight, said that when the man returned to the plane, he ran toward the back. It was not clear how the man had managed to board again.
At one point, the authorities and medics surrounded the man and gave him tissues for his mouth, which was bleeding, Mr. Anspach said. Eventually the man moved to the front of the plane and collapsed sideways into a seat before being taken off the plane on a stretcher, Mr. Anspach said.
In a statement, Oscar Munoz, the chief executive of United Airlines, called the episode “an upsetting event.”
He said the company apologized for having to reaccommodate the customers.
“Our team is moving with a sense of urgency to work with the authorities and conduct our own detailed review of what happened,” he said. “We are also reaching out to this passenger to talk directly to him and further address and resolve this situation.”
In a statement, United said, “We apologize for the overbook situation.”
Andrew D. Gilman, the chief executive of CommCore Consulting Group, a crisis communications firm, said the situation would probably make people reconsider whether they wanted to fly United. That could be particularly damaging with business passengers, a lucrative group that is “outraged as much as anybody,” he said.
“As somebody who flies hundreds of thousands of miles, I’m saying, ‘I thought once I was on the plane, it was my seat,’” Mr. Gilman said. “They unfortunately disrupted a number of certainties that people tend to rely upon, so I think it’s a big trust thing.”
Mr. Gilman said the episode was “also going to be really hard on their employees for a while.”
“It’s a hard enough job — high stress, tense people, delays — and now you have people who are suspicious of you,” he said. “How they can communicate that they were sincere and meant well is going to be very challenging.”