LOUISVILLE, Ky. (WDRB) -- Chicago's aviation department says one of its police officers involved in dragging a man off a United Airlines flight at O'Hare International Airport did not follow standard operating procedures and has been placed on leave.

The department said in a brief statement Monday it did not condone the aviation security officers' actions Sunday night.

The statement did not release the officer's name and it was not immediately clear which of the three men seen in the now-widely seen video taken by another passenger which one was placed on leave.

United has said that the incident came after the flight to Louisville, Kentucky, was overbooked and airline officials asked for volunteers to get off the plane. When none of them did, the airline told four passengers who were selected at random that they had to get off the plane. 

John Klaassen was on the flight, headed back to Louisville from a mission trip in Greece. He was sitting in the row in front of where the incident occurred. He says an United employee boarded the plane and told passengers they needed volunteers to make room for airline employees on standby. 

"They said that they had four airline United personnel that needed to get to Louisville ... She said that they were going to force people off the plane," Klaassen said.

Three passengers selected to exit the plane, but the fourth refused. United called the airport police, who came aboard the plane and dragged him away.

"He said that he had patients that he needed to see in the morning, that he was a medical doctor and that he could not miss this flight," Klaassen said. "The next thing you know, they are forcibly removing him from the plane."

At that point, the now-viral video shows the situation escalating.

"He hits his head, there's blood, they drag him, literally drag him off the airplane," Klaassen said. "We're all just sitting there in bewilderment asking the question 'what just happened?'"

The CEO of United Airlines released a statement earlier Monday after a video showing a man forcibly removed from a flight surfaced on social media Sunday night.

"This is an upsetting event to all of us here at United," wrote Oscar Munoz, CEO of United Airlines. "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."

The video, posted from a Facebook account assigned the name Audra D. Bridges, appears to show authorities surrounding a man in a seat. The man screams in pain, before he is dragged down the aisle of the aircraft, as passengers protest.

"Please share this video," the Facebook caption reads. "We are on this flight. United airlines overbooked the flight. They randomly selected people to kick off so their standby crew could have a seat. This man is a doctor and has to be at the hospital in the morning. He did not want to get off. We are all shaky and so disgusted. #unitedairways"

A statement issued Monday morning by Charlie Hobart, a spokesman for United Airlines, corroborates some of what was posted on social media:

"Flight 3411 from Chicago to Louisville was overbooked," the statement read. "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. Further details on the removed customer should be directed to authorities."

Still, passengers on the plane are expressing concerns about how the situation was handled by United. 

"United is responsible for what took place on that flight," Klaassen said. "There was a real lack of respect for life and a real lack of respect for dignity." 

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