S4, E8: When a young officer is shot dead in the lobby of an apartment building, Intelligence looks to the latest known offenders who have threatened the police. But when a second officer is killed, the team finds that they are dealing with someone who is highly trained and knowledgeable. Voight (Jason Beghe) works to keep the case contained as they deal with Chicago's ever present war against the police. Meanwhile, Lindsay (Sophia Bush) confronts Antonio (Jon Seda) about his new job with the State's Attorney.