CHICAGO (CBS) — United Airlines has claimed a passenger who was dragged off an overbooked flight at O’Hare International Airport on Sunday was being “disruptive and belligerent,” and defended employees who called security, but the company said “there are lessons we can learn from this experience.”

Passengers posted videos of the incident on social media as a Chicago Department of Aviation officer forcibly removed the man from his seat and dragged him off the plane by his arms. The officer has since been placed on leave, and the department has said his actions “are obviously not condoned.”

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The crew on United flight 3411 was trying to make room for four employees of a partner airline who needed to get to Louisville by Monday morning to crew another flight, and had asked for four volunteers to give up their seats.

United first offered $400 and a free hotel stay, and when no volunteers came forward, upped the offer to $800. When they still got no volunteers, a United manager picked four passengers at random.

A man who said he was a doctor and needed to get home to treat patients on Monday refused to get off the plane when he was picked at random, and the flight crew called Chicago Aviation Department officers to help remove the man from the plane.

After the man refused to leave voluntarily, one of the officers pulled the man out of his seat. The man began screaming, and then went silent as the officer dragged him down the aisle by his arms, the man’s glasses knocked askew.

In a statement on Twitter, United CEO Oscar Munoz later 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.

However, in a letter to employees, Munoz defended the flight crew’s actions.

“As you will read, this situation was unfortunately compounded when one of the passengers we politely asked to deplane refused and it became necessary to contact Chicago Aviation Security Officers to help. Our employees followed established procedures for dealing with situations like this. While I deeply regret this situation arose, I also emphatically stand behind all of you, and I want to commend you for continuing to go above and beyond to ensure we fly right,” Munoz said.

According to the CEO’s letter, when gate agents learned four passengers would need to be removed from the flight, they first sought volunteers by offering up to $1,000 in compensation.

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“When we approached one of these passengers to explain apologetically that he was being denied boarding, he raised his voice and refused to comply with crew member instructions,” Munoz said. “He was approached a few more times after that in order to gain his compliance to come off the aircraft, and each time he refused and became more and more disruptive and belligerent.”

United said the gate agents had no choice but to call for security officers to assist in removing the man from the flight when he repeatedly declined to leave voluntarily.

Airlines have the right to remove passengers from a flight when it is overbooked. The terms are usually written in the fine print on tickets, but many observers believe things went too far when security officers dragged the man off the flight.

“That gentleman was not committing any kind of crimes on the flight. He had a ticket, and had boarded like everyone else, and it was just very uncalled-for, and there were kids on that flight. No one should have had to see any of that,” said fellow passenger Jade Kelley, who was seated across the aisle from the man who was removed.

Aviation consultant and former airline pilot Rob Mark said, while United might have the right to forcibly remove a passenger who is unwilling to voluntarily leave an overbooked flight, it should consider how doing so could affect its reputation.

“Is there an easy way and a difficult way to solve the problem? And let’s try to take the easy way,” he said. “United Airlines is certainly within its rights to deny somebody boarding. It’s the way they did it, though, that was really ugly.”

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

“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.

The Chicago Department of Aviation also has said it is investigating the incident, and the officer who dragged the man off the flight has been placed on leave.

“The incident on United flight 3411 was not in accordance with our standard operating procedure and the actions of the aviation security officer are obviously not condoned by the Department,” Aviation Department spokeswoman Karen Pride said in an email.

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The flight was delayed two hours as a result of the incident, after the man – bloodied by the incident – ran back onto the plane after he was removed, and the airline had to clean the plane before it could continue to Louisville.