In May 2005, Duncan kidnapped 9-year-old Dylan Groene, and his sister Shasta, then 8, from their Coeur d'Alene home and held them for weeks in western Montana. He tortured and sexually abused both children, eventually killing Dylan but Shasta was rescued after seven weeks.
Duncan had targeted the children, watching their home and killing their 13-year-old brother, Slade Groene, their mother, Brenda Groene, and her fiance, Mark McKenzie, in order to take the young siblings.
In state court Monday, Duncan was sentenced to three life terms for those murders, a penalty agreed upon mainly to spare Shasta the trauma of testifying. Shortly after that hearing, U.S. District Judge Edward Lodge sentenced Duncan to three federal life terms, one for kidnapping Shasta, and one each for sexually abusing Shasta and her brother.
"By any stretch of the imagination, your rampage ... exceeded the bounds of human understanding," Lodge told Duncan. "These crimes ... were unfathomable, cruel and sadistic."
Prosecutors say the Federal Bureau of Prisons will determine where Duncan, 45, should be held, a decision that may be influenced by whether California authorities pursue criminal charges against him for the 1997 abduction and murder of a 10-year-old boy.
Duncan, a convicted pedophile originally from Tacoma, Wash., has said his violence was motivated by a need to seek revenge on society.
Offered a final chance Monday to explain his actions or offer remorse to Groene relatives, Duncan was unapologetic and instead quoted a biblical passage stating he cared very little about being judged for his crimes.
Duncan had pleaded guilty to 10 federal crimes stemming from his kidnapping and sexual abuse of the children and the slaying of Dylan. Three of those charges qualified for the death penalty, which the federal jury delivered in August.
Prosecutors have said the law requires the multiple death sentences and life terms, and would act as safeguards if convictions or sentences are overturned on appeal.
Duncan has 10 days to file a notice of intent to appeal his federal sentence.
During his federal death penalty hearing, prosecutors spelled out the grim details of the weeks the children were held captive.
Four jurors who decided that sentence attended the sentencing Monday, as did Steven Groene, the father of Shasta, Dylan and Slade.
Lodge ordered Duncan to pay $100,000 in restitution to cover future costs of counseling and treatment for Shasta, though the judge acknowledged Duncan likely is unable to pay.