Duncan was born in Tacoma, Washington, in 1963. He was convicted of the rape of a fourteen-year-old boy at seventeen and served fourteen years, and was on the sex offender register and under supervision for most of his adult life. He was on bail on a molestation charge in Minnesota when he committed the Idaho killings, having been released on a bond that was later the subject of an inquiry into how it came to be set.
On May 16, 2005 he broke into a house near Coeur d'Alene, Idaho, and beat to death Brenda Groene, forty, her boyfriend Mark McKenzie, and her thirteen-year-old son Slade, with a hammer. He abducted her two younger children, Shasta, eight, and Dylan, nine, and held them for seven weeks at a campsite in Montana, abusing both and killing Dylan. Shasta was recognised by a waitress in a restaurant in Coeur d'Alene on July 2, 2005, when he brought her in; the police were called and he was arrested there.
He also admitted killing Anthony Martinez, ten, taken from Beaumont, California, in April 1997, and Sammiejo White, eleven, and Carmen Cubias, nine, who disappeared in Seattle in July 1996. Charges were brought in the Martinez case; the Seattle killings were confirmed by his own account and by physical evidence at a site he identified.
He pleaded guilty in Idaho and received six life sentences, then pleaded guilty in federal court to the kidnapping and murder of Dylan Groene and was sentenced to death in August 2008. A federal death sentence for a crime on a state that had itself not executed anyone in decades. He waived his appeals, then his lawyers litigated his competence to have waived them, and the case ran for another twelve years. He died of brain cancer at the federal medical centre in Terre Haute on March 28, 2021.
Shasta Groene survived and has spoken publicly about the case as an adult. Six convictions stand in Idaho alongside the federal death sentence, and there are further disappearances he claimed that were never established.