Ghost

S6 E16: A well-to-do couple, the Brevets, own an investments firm, and they are found separately, tortured and murdered. It turns out they were laundering money for South American drug dealers, and skimming some for themselves. Through Detective Sandoval (Nicholas Gonzalez), they find the middleman, Fausto Montoya, and when they go to his apartment, he and his wife are also dead. His eight-year-old son Antonio (Reymond Wittmann) has been shot in the head but miraculously survives, and he tells the cops a ""ghost"" killed his parents. Ballistics matches the slugs from the Brevet murder to the ""murder"" of the SVU's former A.D.A. Alexandra Cabot.