An amazing article from the Chicago Reader describes a recent incident in which an out-of-uniform police officer who was late arriving to work, shot an unarmed man in the head at point blank range, in full view of security cameras.
The officer lied and said that at the time of the shooting he was surrounded by 4 or 5 men who had threatened his life. When police discovered that the video footage existed, the story was changed to say that the victim had raised a fist and attempted to disarm the officer, and that the cop had raised his arm and accidentally shot the victim through the head.
And it gets worse from there. I encourage you to watch the footage and listen to the narration.