Friday's verdict was not surprising after Duncan's closing argument, in which he told the jurors they didn't yet "have a clue" about the depths of his "heinousness."
Duncan, 45, didn't react to the verdict. The slain boy's father, Steven Groene, and other family supporters embraced after the verdict but seemed to keep their emotions in check.
Duncan, acting as his own attorney, will have the chance next week to convince jurors to give him life in prison without the possibility of parole instead of a death sentence. The jury's sentencing recommendation is binding on U.S. District Judge Edward Lodge.
Duncan pleaded guilty to 10 federal felonies for the trauma he inflicted on Dylan Groene and his then-8-year-old sister, Shasta, in May 2005 after murdering their older brother, their mother and her fiance in the Coeur d'Alene area. Duncan pleaded guilty to those three murders in state court, where he also could be sentenced to death.
He kidnapped the two young children and took them deep into the Lolo National Forest, where he sexually abused and tortured them for weeks. He ultimately shot the boy point-blank in the head while his sister watched. He was arrested after returning with Shasta to Coeur d'Alene, where a waitress recognized the girl as the two ate at a Denny's restaurant.
Jurors cringed and cried when they viewed videos Duncan made in which he molested, tortured and hanged Dylan Groene until the boy was unconscious and nearly dead. Duncan told the panel that government lawyers helped him victimize the jurors by making them watch and listen to the evidence.
"I should actually thank the government for helping me get my eye for an eye by showing you the evidence that you've seen, the videos," Duncan said during his closing argument Friday.
Duncan, a convicted pedophile originally from Tacoma, Wash., told jurors that by presenting the evidence, the government was "helping me to take away your heart and your innocence."
"That's what they have done, and I should thank them but I won't," he said.
Steven Groene declined to discuss the case much, saying a gag order remained in effect, but said he was pleased with the ruling.
Groene said he would lobby for stronger victims' rights in the future. He objected to spectators at the hearing being permitted to view the videos of his son being abused, and said the spectators should have been screened to make sure they weren't sex offenders looking to take pleasure from the highly publicized case.
"My murdered, molested son should have had the same rights as a living victim," Groene said. "I've had too much focus on keeping my family together and protecting my daughter, fighting cancer for the last few years but now that this appears to be close to over, I'm going to work on changing that."
Duncan said he wasn't in court because he was caught, but because Shasta Groene - the sole survivor of the kidnapping and attack - didn't judge him for his actions, prompting him to take her home.
It's not yet clear if Shasta will testify - and face the prospect of being cross-examined by her attacker - in the next phase of the trial. She did not testify in the phase just completed.
The court has ruled that if the girl does testify, it will be by closed-circuit camera and that the courtroom will be closed to the public, with a transcript of her testimony given to reporters afterward.
Duncan, originally from Tacoma, Wash., was a convicted pedophile before the killings. The rampage was the culmination of years of planning, he said, and he originally intended to rape and kill until he was killed.
Duncan has a long string of arrests and convictions for crimes ranging from car theft to rape and molestation. He is suspected in the slayings of two half-sisters from Seattle in 1996 and is charged with killing a young boy in Riverside County, Calif., in 1997.
A witness testified for the prosecution on Thursday that Duncan had raped him at gunpoint in 1980, when the man was just 14 years old.
The Associated Press generally does not identify victims of sexual assault. In Shasta and Dylan Groene's cases, however, the search for the children was so heavily publicized that their names are widely known.