The U.S. Department of Transportation has opened an investigation into the treatment of a United Airlines passenger who was forcibly dragged from a plane. DOT said in a statement Monday evening that it was reviewing whether United complied with rules on that require airlines to establish a reasonable procedure on how to deal with passengers if they don’t volunteer to give up their seats: US Dept of Transportation looking into the United incident as well pic.twitter.com/fXujb8HpB5 Several videos show security personnel prying the man out of his seat and dragging him down the aisle of a plane in Chicago shortly before it was to take off on Sunday to Louisville, Kentucky. The man was screaming and his lip apparently bloodied as several people on the plane expressed their disgust at his treatment. @united @FoxNews @CNN not a good way to treat a Doctor trying to get to work because they overbooked pic.twitter.com/sj9oHk94Ik Initial reports said the flight was overbooked. However, United Airlines said in a Tuesday statement to USA Today that the flight was not overbooked, but United and their regional affiliate, Republic Airlines, felt the need to remove four passengers to allow crew members to travel aboard the plane. United CEO Oscar Munoz issued a statement expressing remorse for the incident. But in an email to employees obtained by ABC he said that while the situation “upset” him, the passenger was “disruptive and belligerent.” One of the officers involved in the incident has been suspended, pending an investigation, Chicago’s Department of Aviation security confirmed in a statement. “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,” spokeswoman Karen Pride said. United clarified on Tuesday that the plane was not technically overbooked in a statement to USA Today. This article has been updated accordingly.