Charges announced Tuesday come as the city prepares to release a squad-car video of Officer Jason Van Dyke shooting 17-year-old Laquan McDonald 16 times on Oct. 20, 2014.
People who have seen the video say it shows McDonald armed with a small knife and walking away from officers. Van Dyke opens fire from about 15 feet and keeps shooting after the teen falls to the ground.
An autopsy says he was shot at least twice in the back.
Some worry the images could lead to unrest as in Baltimore and Ferguson, Missouri, after police-involved deaths.
Tuesday evening, police released a video of the shooting. Warning: some may find the video too graphic.
