On Sunday, a United Airlines passenger was pulled from his plane seat and forcibly dragged off the aircraft — due to the airline overbooking the flight.

At least two passengers onboard captured the scene, which shows a man-- who has not been identified-- being forcibly removed from his seat and dragged down the aisle as others look on in apparent disbelief. Some clips appear to show that the man was left bleeding from the head after his face was smashed against an armrest during the scuffle.

On Monday, Chicago Police confirmed they were not involved in the incident and that the officers and security personnel seen in the now viral videos are with the Department of Aviation, a division of the U.S. Department of Transportation.

MAN KICKED OFF FLIGHT FOR 70-POUND TRAVEL COMPANION

The man, who some social media users have said is a doctor, lets out a scream during the ordeal. One passenger yells, "No, this is wrong!"

@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

“Look at what you did to him,” another person exlcaims as the man is dragged.

According to the New York Post, the incident occurred on a United Airlines flight that later departed Chicago O'Hare International Airport for Louisville, Ky.

Tyler Bridges tweeted, "Not a good way to treat a doctor trying to get to work because they overbooked."

"He told the police and the United employees he had to be at the hospital in the morning to see patients," Bridges further explained.

Before the flight departed, Bridges said passengers had been told at the gate that the airline was offering $400 and a hotel stay for any volunteer who would take a flight at 3 p.m. the next day. After passengers boarded, however, a crewmember alerted fliers that people would have to give up their seats to United employees who needed to be in Kentucky for work, Bridges told the Courier-Journal.

Even though United upped the offer $800, and the airline announced that the flight wouldn't depart until the crewmembers were situated, no one budged.

That's when security officers reportedly boarded the plane. Initially, the Post reports, one couple was asked to leave-- they both complied-- and then the man seen in the video was "confronted."

On Monday, the airline's official Twitter account posted a statement from United's CEO Oscar Munoz, who called the event "upsetting" and apologized for "having to re-accommodate these customers."

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

Under to United's Contract of Carriage (or rules for passengers), "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."

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Several factors come into play when an airline decides who to remove, including 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." Those with disabilities or children traveling alone are least likely to be asked to leave.

A representative for United Airlines was not immediately available for comment.

Just last month, United came under fire for barring teenage passengers from a flight for wearing leggings. Though the airline explained that the young passengers were flying on free friends-and-family passes, which require guests to follow a stricter dress code, many on social media-- including celebrites like Chrissy Teigen and William Shatner-- criticized the policy.

UPDATE: On Monday afternoon, Chicago's aviation department confirmed that one of its police officers involved in dragging the passenger from his seat did not follow standard operating procedures and has been placed on leave. The department said in a brief statement that it did not condone the aviation security officers' actions. The name of the officer has not been released.

The Associated Press contibuted to this report.