News of a man being forcibly removed from a United Airlines flight on Sunday after not voluntarily giving up his seat is making the rounds, raising questions about what authority airlines have to remove ticketed passengers in situations of overbooking.

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According to accounts from passengers on the flight, which was leaving from Chicago O'Hare International Airport and bound for Louisville, the airline wanted the seats for employees who needed to travel to be at work the next day. Cellphone video from the aircraft shows a man who said he was a doctor being forced from his seat and dragged down the aisle of the plane as onlookers screamed, "Oh, my God!"

Related: Man forcibly removed from flight after not voluntarily giving up seat on overbooked flight

VIDEO: Passenger Removed From United Airlines Flight

It hasn't been a great few months for United Airlines. In March, the airline received widespread criticism for barring two teens from their flight because they were wearing leggings.

So in what situations do the airlines have the right to force ticketed passengers from a plane? And what is the protocol for doing so?

According to the U.S. Department of Transportation, overbooking is legal, with most airlines overbooking their scheduled flights to a certain extent in order to compensate for no-shows. When overselling occurs, the DOT requires airlines to ask people to give up their seats voluntarily in exchange for compensation. If no one volunteers, the airline may then bump passengers involuntarily, although they too are entitled to compensation.

According to United's Contract of Carriage, "If a flight is oversold, no one may be denied boarding against his/her will until UA or other carrier personnel first ask for volunteers who will give up their reservations willingly in exchange for compensation as determined by UA. If there are not enough volunteers, other passengers may be denied boarding involuntarily in accordance with UA's boarding priority."

The contract states that passengers with disabilities, unaccompanied minors under the age of 18 and minors ages 5-15 who use the unaccompanied minor service will be the last to be involuntarily denied boarding. It adds that “the priority of all other confirmed passengers may be determined based on a passenger’s fare class, itinerary, status of frequent flyer program membership, and the time in which the passenger presents him/herself for check-in without advanced seat assignment.”

According to the DOT's Consumer Guide to Air Travel, airlines must give all passengers who are bumped involuntarily "a written statement describing their rights and explaining how the carrier decides who gets on an oversold flight and who doesn't. Those travelers who don't get to fly are frequently entitled to denied boarding compensation in the form of a check or cash. The amount depends on the price of their ticket and the length of the delay."

DOT statistics show that, on average, only about one of every 10,000 airline passengers is bumped involuntarily, although that number can increase over the holidays and during other busy travel seasons.

United has said little about the incident but did release this response to WHAS: "Flight 3411 from Chicago to Louisville was overbooked. After our team looked for volunteers, one customer refused to leave the aircraft voluntarily and law enforcement was asked to come to the gate. We apologize for the overbook situation."

United CEO Oscar Munoz later issued a statement on Twitter Monday, saying,  "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."