The jurors' recommendation was binding on U.S. District Judge Edward Lodge, who thanked them, dismissed them and then sentenced Joseph Edward Duncan III.
Relatives of the victim, Dylan Groene, remained somber as the jury's decision was announced. Duncan murdered Dylan's mother, older brother and his mother's fiance to kidnap him and his younger sister, who was sexually abused along with her brother but survived.
Duncan showed no reaction other than smiling as the verdict was passed to the judge.
He took Dylan and the boy's then-8-year-old sister, Shasta, to a remote western Montana campsite where he raped, tortured and threatened them before shooting Dylan in the head and burning his body. Jurors viewed horrifying video Duncan made of him sexually abusing, torturing and hanging Dylan until the boy lost consciousness.
"This defendant is dangerous. He is a predator who takes pride in his work," prosecutor Traci Whelan said. "He earned this day. His actions ... call out for the death penalty."
Duncan acted as his own attorney but had offered no response to prosecutors' closing argument.
"I have no argument," he told the court.
With an eye toward kidnapping the two children, Duncan stalked their family. In 2005 he entered their Coeur d'Alene-area home and used a hammer to fatally bludgeon their 13-year-old brother, Slade Groene, his mother, Brenda Groene, and her fiance, Mark McKenzie.
Duncan was arrested and Shasta rescued weeks after the kidnappings when a waitress at a Denny's in Coeur d'Alene called police after recognizing the two as they ate.
Duncan pleaded guilty in December to 10 federal charges involving the kidnappings and the murder of Dylan. He pleaded guilty to the other three murders in state court, where he also could be sentenced to death.
"I am so glad this is over. Justice has been served," said Darlene Torres, Brenda Groene's mother. "It's been very painful."
She said that when she saw Duncan in court, "I seen nothing but an evil, empty, coldhearted shell."
In closing arguments, Whelan reminded the jury of Duncan's lifelong "pattern of violence," including a conviction for raping a boy at gunpoint in 1980. Duncan has told investigators he killed two half-sisters from Seattle in 1996, and he is charged with killing a young boy in Riverside County, Calif., in 1997.
Duncan may now be brought to Riverside County to stand trial in the death of Anthony Martinez.
Whelan told the jury that Duncan would pose a risk even to prison guards and fellow inmates.
"This defendant uses the time that he has to think out these plans and he is dangerous," she said. "He is adaptable and he is dangerous."
The heinousness of the evidence in Dylan's murder made it particularly difficult for the jurors to remain impartial as they deliberate, said Art Patterson, a jury consultant and senior vice president of the trial consulting firm DecisionQuest.
"Generally, for human beings, it's pretty hard to maintain impartiality when confronted with such horror," Patterson said.
"How could any juror not want to see this person removed from our list of living human beings? How could you live with yourself as a juror if there's any chance this human being could escape from jail and do something like this again?" Patterson said.