Mesmerizing, mournful portrait of serial-killer/rapist Arthur Shawcross--who also practiced necrophilia and cannibalism--that digs deep to lay his tortured psyche bare. Olsen has profiled numerous madmen before (Predator, 1991, etc.) but rarely with such diligence--or one so heinous. He presents Shawcross's story in oral-history form, binding together testimony from the killer (mostly Q&A transcripts), cops, psychiatrists, relatives of Shawcross's victims, etc., with his own extensive narration. An expert storyteller, Olsen begins with high melodrama: the disappearance in 1972 in Watertown, New York, of ten-year-old Jack Blake--Shawcross's first victim, raped, mutilated, and killed. Despite the insistence of Jack's mother that neighbor Shawcross--then a notably eccentric 27-year-old Vietnam vet--had slain her boy, it took a second killing, of a local girl, to put Shawcross behind bars for a presumed 25 years. But after 14 years, the killer convinced a parole board of his rehabilitation and was freed. Moving to Rochester and marrying a prison pen-pal, Shawcross went on a years'-long spree of killing prostitutes. Dogged police work and a lucky break finally did him in. Olsen closely details Shawcross's gruesome crimes and the cops' counterpoint, but his focus is on motivation: What made Shawcross kill? The author excavates the murderer's early years, uncovering an unhappy home but no striking abuse; explores Shawcross's own rational--that he became a killer in Vietnam--and finds his stories of jungle savagery to be tall tales; and locates only a few clues in Shawcross's accounts of his murders. The unexpected answer is revealed at book's end, in testimony from a psychiatrist who discovered behind the killer's compulsion a terrible biological imperative: an extra Y chromosome and a rare chemical imbalance. Olsen explains Shawcross without excusing him, creating an unforgettable portrait, horrifying yet compassionate, of a doomed modern-day monster.