CHICAGO -- An investigation is underway after a Chicago police officer was apparently caught on cellphone video punching a man he was trying to arrest.In the video, the officer has the man pinned down to the ground as he punches him at least four times in the head. The incident occurred Sunday near 88th and Wabash."The guy's down. His hands was not swinging up, resisting, throwing blows or trying to flip the officer off. He was not," says Maurice Fulson who recorded the encounter.Fulson says he started filming after hearing a commotion Sunday afternoon.According to police, officers were trying to ticket 22-year-old Robert Foreman for public alcohol consumption, but he ran and resisted arrest. In the video, Foreman says he had just been the victim of a robbery."How are you going to come at me? I was just getting robbed! I was just getting robbed!" Foreman shouts in the video.Chicago police have issued a response, saying in part:"Based on this video, CPD has opened an internal affairs investigation into this incident and will be forwarding the video and all relevant reports to the Independent Review Board for investigation."This incident comes one week after a video surfaced of another Chicago police officer kicking a man in the head. The suspect was later charged with aggravated battery of an officer."When we make honest mistakes, or sometimes just the day-to-day things that police do, it's not going to look pretty all the time," Supt. Eddie Johnson said at the time.Foreman's attorney maintains his client was the victim of a robbery and had no reason to run from police or resist arrest. Police say the officer in question has not been placed on desk duty and remains in the field.Foreman's attorney says his client is considering taking legal action.