Poison (S01E07)

When a mysterious "angel of death" poisons a series of hospital victims with cyanide, Detectives Goren (Vincent D'Onofrio) and Eames (Kathryn Erbe) suspect a nurse (guest star Susan Bruce) who fits the typical profile – but when others fall dead outside the care facility, the investigators must widen their search. Even when the cops discover the source to be tainted, over-the-counter painkillers, the victims continue to pile up, and they can't gain any traction with their case until they find a motive – or at least a source of the killer substance. (iTunes)