“He came up. He had blood all over him, and he said they were all dead.” Those are the words 16-year-old Jody Gilley said to the 911 operator after her 18-year-old brother, Billy, beat their mother, father and pre-teen sister to death with an aluminum baseball bat. The only family member Billy let live was Jody, whom he claimed to be in love with. In her new nonfiction book While They Slept (Random House, 285 pages, $25), author Kathryn Harrison pulls the reader through the story of the 1984 triple murder in Medford—our own backyard—with such speed and excitement it feels like you’re watching an episode of Law & Order: Criminal Intent, without the proverbial “dun-dun” at the end of each chapter.

Harrison is no stranger to writing about abusive situations. Her 1998 memoir, The Kiss, chronicles her own sexual abuse inflicted by her father when she was 20 years old. In While They Slept, Harrison provides a narrative of the Gilley murders using information that had, until now, been lost in a confusing hodgepodge of witness testimonies, news reports, psychiatric evaluations and social-service records. After 18 hours of interviews with Billy, who was sentenced to three consecutive life sentences and is incarcerated at Jackson County Jail in Medford, and extensive email and in-person interviews with Jody, a successful communications strategist in Washington, D.C., Harrison re-creates each moment of the murders and their aftermath.

Harrison perfectly paces the revelations of new characters, who add critical information and perspective to the Gilley murder. Jody’s best friend and confidante, Kathy Ackerson, Billy’s laundry list of psychiatrists and social workers who identified his anger problems and impulsiveness at an early age, and extended family members add layer after layer to the story, literally thickening the plot.

Harrison’s first-person commentary is not overly self-indulgent; in fact, it adds smart insights that readers wouldn’t be able to conclude from their cursory encounters with the Gilley’s family story. The crates full of documents in Harrison’s office (which she appropriately dubbed “Gilleyalia”) flesh out the Gilley family history: Physical beatings, verbal abuse, distrust and deception were everyday atrocities in Bill and Linda Gilley’s household. “You’re going to grow up to be a man, not a sissy, even if I have to kill you,” was Bill’s encouragement for Billy as the young boy learned to swim. According to Harrison, the Gilleys were a tag-team of fierce tyrants complicit in each other’s torment of their three children—a conclusion the author draws after listening to Billy and Jody’s vivid accounts of their parents’ abuse.

It’s those interviews that shape the judgments and deductions Harrison makes about the Gilleys throughout the book. But Harrison’s personal admission that “I trust Jody, as I don’t her brother” poses a real challenge to the book’s objectivity, as she takes Jody’s word over Billy’s on almost every divisive issue.

From page one of While They Slept, it’s evident the Gilley family had a grim forecast. So grim that Harrison’s thorough analysis reveals Billy’s triple murder as no surprise, but inevitable and even, perhaps, understandable.