CHICAGO (CBS) — A Chicago Aviation Department police officer was placed on leave Monday, after he forcibly removed a United Airlines passenger from his seat and dragged him off an overbooked flight on Sunday.

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 International Airport to Louisville, Kentucky, on Sunday.

The man could be heard screaming when one of 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.

The U.S. Department of Transportation confirmed 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.

@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

Monday afternoon, Aviation Department spokeswoman Karen Pride said the officer who dragged the passenger off the plane has been placed on leave, pending an investigation.

“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,” Pride said in an email.

Apparently bleeding from the incident, the man somehow got back on the plane about 10 minutes after he was removed, 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.

United CEO response to United Express Flight 3411. pic.twitter.com/rF5gNIvVd0 — United (@united) April 10, 2017

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.”

Other passengers on the flight said United crew first told them they needed a passenger to give up his or her seat, because the airline needed to get an employee on standby to Louisville by Monday morning. However, passengers were allowed to board, and when the flight was filled, the crew said four seats were needed for standby employees.

Passengers said the crew first offered $400 and a hotel stay to anyone willing to give up their seat, but when there were no takers, offered $800. When there still were no volunteers, a manager picked four passengers at random to be removed from the flight.

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

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