"That officer is in a very, very tough place; unimaginable what that officer has to be feeling right now," Chicago Police Supt. Larry Snelling said.

Chicago police officials say an officer killed this week was accidentally shot and killed by her partner after they encountered someone with a rifle when they chased a suspect into an apartment.

Chicago police Officer Krystal Rivera was mistakenly shot and killed by a fellow cop during a confrontation with an armed suspect late Thursday, according to police, who continue to hold at least