BOISE, Idaho (AP) - A man accused of kidnapping two Idaho children, killing one of them, after slaying their family, has confessed to the killings of three other children a decade ago in Washington state and California, federal prosecutors said Tuesday.

The prosecutors cited the confessions to the old killings in court papers saying that they 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, who lived in Fargo, N.D., before his arrest, 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 say 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, Calif., in April 1997, as his friends looked on.