A plainclothes Chicago police officer was caught on a cell phone video (below) kicking a black man in the head on June 13.
Terrance Hobson, who filmed the incident, told NBC Chicago: "The officer was punching him at first, when they first got on the ground and what you guys saw on the video was him trying to move his hands" to stop the punching.
Hobson added that the first officer asked the second officer to help him: "The officer came over there and kicked him in the face."
After Hobson's video went viral on Facebook June 13, the Chicago Police Department said in a statement: "Upon viewing the video, Superintendent [Eddie] Johnson immediately asked the Independent Police Review Authority to investigate the matter while the department gathers more information about the case."
"The Chicago Police Department holds its members to the highest possible professional standards with a priority on treating all Chicagoans with respect."
The video shows the first plainclothes cop trying to arrest the man, Shaquille O'Neil, who was reportedly fleeing and resisting.
The second officer arrives on the scene and yells, "Get back! Get back!" at bystanders. He speaks into his radio, and then yells at witnesses, "Yeah, I am calling back up!" He then walks over to O'Neil and kicks him in the head.
After witnesses gasp in shock, the second officer yells at them: "Get the f--- back! Get the f--- back!"
WGN notes that O'Neil appears to be unconscious, is handcuffed, but does not receive any medical assistance until paramedics arrive on the scene.
The Chicago Fire Department said that the 24-year-old man was unconscious and was transported to a local hospital where he was in fair condition on the night of June 13.
"I don’t know what he did, the guy that they were detaining," Hobson told NBC Chicago. "I don't know what he did wrong, but everybody [saw] what you did and that was wrong too."
O'Neil's mother said that she doesn't know what her son is being treated for at the hospital.
The Chicago Police Department has not said why the man was being arrested but did say charges are forthcoming, notes the Chicago Tribune.
The officer who kicked O'Neil has been relieved from active duty during the investigation, a move that is opposed by his union - the Fraternal Order of Police.
WARNING: GRAPHIC LANGUAGE, VIDEO