S1, E5: When two off-duty cops are involved in a shooting outside a gym, Frank quickly turns from protecting them from I.A. to investigating them for wrongdoing himself. He soon discovers the two cops are dealing steroids. But when investigators get close, a friend of Frank's is killed. Frank blames himself for the tragedy and will now stop at nothing to bring down the rogue cops, even if he has to use his dead friend as bait to set a trap.