CHICAGO (CBS) — The CEO of United Airlines has apologized, after a man was dragged off an overbooked flight at O’Hare International Airport, when he refused to get off the plane voluntarily as the crew tried to find seats for airline employees who needed to get to Louisville.

Other passengers posted videos of the incident on Twitter and Facebook as three Chicago Aviation Department police officers forcibly removed the man from his seat on Flight 3411 from O’Hare to Louisville on Sunday.

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The man could be heard screaming when the officers grabbed him, and then he went silent as he was dragged down the aisle, his glasses knocked down his face, and his shirt riding up his torso. One woman said, “Oh my God. Look at what you did to him” as the man was taken off the plane.

Apparently bleeding from the incident, the man somehow got back on the plane a short time later, repeatedly saying “I have to go home” as he ran up the aisle to the back of the plane.

United CEO Oscar Munoz said the incident was “an upsetting event to all of us here at United.”

“I apologize for having to re-accommodate these customers. 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,” Munoz said in a post on Twitter.

The U.S. Department of Transportation confirmed it is looking into the incident to determine “whether the airline complied with the oversales rule.”

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“The Department is responsible for ensuring that airlines comply with the Department’s consumer protection regulations including its oversales rule. 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 prepared statement issued by the agency said.

United Airlines spokeswoman Megan McCarthy said the flight was overbooked, and the crew asked for volunteers to give up their seats.

“After our team looked for volunteers, one customer refused to leave the aircraft voluntarily and law enforcement was asked to come to the gate,” McCarthy said in an email. “We apologize for the overbook situation. Further details on the removed customer should be directed to authorities.”

Fellow passenger Audra Bridges, who posted video of the incident on Facebook, told the Courier-Journal in Louisville that passengers were offered $400 and a hotel stay to take another flight after they were informed the flight was overbooked, but passengers were allowed to board.

She said, after the flight was filled, passengers were told four people needed to give up their seats so United employees on standby could get to Louisville for another flight on Monday, and the plane would not take off until those airline employees had seats. United then offered $800 to volunteers, but no one accepted, and a manager picked four people at random, according to Bridges.

Bridges’ video of the incident has been shared more than 25,000 times and viewed more than 2 million times on Facebook.

Other passengers said the man who refused to leave repeatedly he was a doctor who needed to see patients on Monday.

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The flight was delayed two hours as a result of the incident.