Duncan, 44, appeared in U.S. District Court in Boise, where he changed his pleas to 10 federal charges, three of which carry the potential of the death penalty. Judge Edward Lodge accepted the plea and ordered a jury to begin deciding Duncan's fate on Jan. 28.
"I just wanted to say that since my arrest, I have never made any attempt to deny my responsibility," Duncan is quoted in transcripts of the hearing.
Federal prosecutors, potential witnesses and Duncan's lawyers all refused to comment on the plea, citing a gag order placed on them at the time of Duncan's arraignment early this year.
Duncan, appearing with long, flowing hair in contrast to his clean-shaven appearance when he pleaded guilty to state charges in 2006, confessed to Lodge that federal prosecutors had all the facts right to convict him of kidnapping Dylan Groene, 9, and his then-8-year-old sister Shasta, early on May 16, 2005 from the home southeast of Coeur d'Alene, then spiriting them across the Montana border and into the Lolo National Forest near St. Regis. There, he sexually molested the children before shooting the boy to death and leaving his body there.
Duncan, who lived in Tacoma before serving a 20-year prison sentence for raping a teenager at gunpoint, now faces sentencing on capital charges of kidnapping Dylan, resulting in the boy's death, sexual exploitation of the child resulting in his death, and using a firearm in commission of a crime of violence resulting in death.
It's not immediately clear if the girl, the sole survivor and witness to her brother's death, will have to testify in the penalty phase. In that phase, jurors will be asked to decide if Duncan deserves to die for his crimes.
The remaining charges against Duncan are kidnapping Shasta, aggravated sexual abuse of both children, possession and transportation of a stolen, unregistered firearm and transportation of a stolen vehicle.
In October 2005, Duncan pleaded guilty to state charges of kidnapping and killing the children's mother, brother and mother's boyfriend, after staking out the house for days.
Idaho 1st District Judge Fred Gibler sentenced Duncan to three consecutive life terms without the possibility of parole for kidnapping the trio before killing them, but stayed, at the state's request, sentencing on the murder counts to give federal prosecutors a chance to try him.
In announcing that deal, Kootenai County Prosecutor Bill Douglas said if Duncan were to escape a death sentence in federal court, Duncan would return to Coeur d'Alene to face a jury in a death penalty phase.
Duncan was arrested early in the morning of July 2, 2005, while eating at the Coeur d'Alene Denny's Restaurant. With him was Shasta.
Duncan claimed he was simply returning the girl to her father, but officials close to the case said they speculate Duncan was feeding the girl a "last supper" before killing her, then dumping the body somewhere close to home, then leaving.