Chicago PD Season 4 Episode 4: Big Friends, Big Enemies
When a car plows through a crowd of concert goers and gunfire erupts, Lindsay and Ruzek are on the scene. Unsure of the motive and concerned a terrorist group may be responsible, Voight orders in Homeland security, customs and the F.B.I. to assist.
Watch it here : Chicago PD Season 4 Episode 4