The prosecutors cited the confessions to the old killings in court papers saying that intended to seek the death penalty against Joseph Edward Duncan III, who was indicted last week on charges involving the two northern Idaho children.
"The defendant has engaged in a continuing pattern of violence, attempted violence, and threatened violence," prosecutors said. Duncan "is likely to commit criminal acts of violence in the future that would constitute a continuing and serious threat to the lives and safety of others."
Roger Peven, Duncan's attorney, said he had not yet seen the filing and could not immediately comment on it.
Duncan is accused of kidnapping Dylan Groene, 9, and his sister Shasta, then 8, in May 2005, and taking them to the mountains of Montana, where prosecutors say he sexually abused them for weeks before killing Dylan.
Duncan was arrested July 2, 2005, when he and Shasta were spotted at a Coeur d'Alene restaurant. The boy's body was found days later at a remote campsite.
The U.S. attorney's office said Duncan confessed that he killed Carmen Cubias, 9, and Sammiejo White, 11, in Washington state in 1996 and Anthony Martinez, 10, in California in 1997. Officials did not specify to whom Duncan made the confession.
The two girls were kidnapped from the Crest Motel in Seattle in July 1996. Their skeletal remains were found 17 months later in Bothell, a Seattle suburb.
Anthony was forced into a car in Beaumont, California, in April 1997 as his friends looked on. Sixteen days later a forest ranger found the boy's nude, bound body about 70 miles to the east.
Last October, Duncan pleaded guilty in Idaho state court to first-degree murder and kidnapping for the May 16, 2005, hammer slayings of Dylan and Shasta's mother, Brenda Groene; her fiance, Mark McKenzie; and Groene's 13-year-old son, Slade. Prosecutors say he killed them to get the younger children.
If federal prosecutors fail to win a death sentence in the case involving the two younger children, a jury will be chosen in Idaho state court to consider whether to impose the death penalty on the murder counts that Duncan pleaded guilty to in October.
Duncan was charged Thursday in a California state court in Anthony's death. Prosecutors there said they also intend to seek the death penalty.
Duncan is a Tacoma, Washington, native who spent most of his adult life in Washington state prisons for sexual crimes against children.
In 2004, he had been arrested for allegedly molesting a 6-year-old boy and attempting to molest another boy in Detroit Lakes, Minnesota. Authorities say he jumped bail on that charge.
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