United Airlines says the passenger who was violently dragged off one of its planes refused to leave after no one volunteered to give up their seat on the overbooked flight.



A series of videos posted on Twitter Sunday evening show officers forcibly removing a man from his seat and dragging the distressed passenger down the aisle by his wrists, as other customers yelled out in protest.



“After our team looked for volunteers, one customer refused to leave the aircraft voluntarily and law enforcement was asked to come to the gate,” a United spokesperson said. “We apologize for the overbook situation.”



United’s CEO Oscar Munoz later said in a statement that the company is conducting a detailed review of the “upsetting event.”



“This is an upsetting event to all of us here at United. I apologize for having to re-accommodate these customers,” Munoz said. “Our team is moving with a sense of urgency to work with the authorities and conduct our own detailed review of what happened.”



“We are also reaching out to this passenger to talk directly to him and further address and resolve this situation,” he added.

@united @FoxNews @CNN not a good way to treat a Doctor trying to get to work because they overbooked pic.twitter.com/sj9oHk94Ik — Tyler Bridges (@Tyler_Bridges) April 9, 2017

@United overbook #flight3411 and decided to force random passengers off the plane. Here's how they did it: pic.twitter.com/QfefM8X2cW — Jayse D. Anspach (@JayseDavid) April 10, 2017

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The incident , which sparked widespread outrage, is also raising questions about the overbooking and bumping process for airlines, which is common and legal.One passenger said on Twitter that the United flight from Chicago to Louisville was overbooked by four people, with those seats needed for airline personnel. But after no one volunteered to leave, a manager said passengers would be randomly selected to give up their seats.The man who was dragged off the plane was apparently one of the customers selected at random, but refused to leave because he said he was a doctor and needed to reach his destination to treat patients. Officers were then brought on board to assist with removing him.The Department of Transportation (DOT) says most airlines overbook their scheduled flights “to a certain extent” in order to compensate for no-shows. When that occurs, airlines are required to first offer volunteers compensation in exchange for their seats before bumping someone involuntarily.Airlines bumped 0.62 customers per every 10,000 passengers last year, which is the lowest annual rate based on historical data collected since 1995, according to the latest DOT data available.

The DOT said it is reviewing the latest incident to ensure United complied with all consumer protection rules.



“While it is legal for airlines to involuntary bump passengers from an oversold flight when there are not enough volunteers, it is the airline’s responsibility to determine its own fair boarding priorities,” a DOT spokesperson said in a statement.



Anyone bumped against their will may be entitled to compensation, the agency says, and must be given a written statement detailing their rights and explaining how the airline decides who gets on an overbooked flight and who doesn't.



“Airlines set their own ‘boarding priorities’ — the order in which they will bump different categories of passengers in an oversale situation,” the DOT says on its website. “When a flight is oversold and there are not enough volunteers, some airlines bump passengers with the lowest fares first. Others bump the last passengers to check-in.”



It’s unclear whether passengers on the United flight were first offered compensation and how customers were selected to be bumped off the flight. The airline did not immediately return a request for comment.

--This story was updated at 1:18 p.m.