S4, E10: After the body of young woman is found with multiple gunshot wounds, Intelligence uncovers details of her case that are similar to other deaths. They work to link all of these murders together to pin down the serial killer, and appease the families of the victims who have been looking for answers. Meanwhile, Lindsay (Sophia Bush) decides to meet her biological father (guest star Tony Crane) for the first time, and Voight (Jason Beghe) welcomes Kenny Rixton (guest star Nick Wechsler) to the team when Ruzek (Patrick John Flueger) takes an undercover assignment.