The thug who physically removed the Doctor from the flight was employed by the airport security services, he was an ex policeman and had history. He was not a serving police officer, nor was he a United Airlines employee.
Actually, they ARE sworn police officers. (According to the Chicago Tribune article from today that I can't link). They just aren't allowed to carry guns. They are their own agency, but, according to the article, function under the command umbrella of CPD, TSA, ICE, and
Federal Protective Services.