Boise, Idaho ? A man accused of kidnapping and molesting two Idaho children after killing their family has confessed to the slayings of three other children in Washington state and California, federal prosecutors said Tuesday.

In court documents, the prosecutors cited the confessions to the old killings as they argued to seek the death penalty against Joseph Edward Duncan III, who was indicted last week on charges of kidnapping two northern Idaho children and killing one of them.

“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 seen the filing and could not comment.

Duncan is accused of kidnapping Dylan Groene, 9, and his sister Shasta, then 8, and taking them to the mountains of Montana, where he sexually abused them for weeks before killing Dylan. The boy’s body was found at a remote campsite.

Duncan pleaded guilty last October 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.

The U.S. attorney’s office said Duncan confessed to killing Carmen Cubias, 9, and Sammiejo White, 11, in Washington state in 1996 and Anthony Martinez, 10, in California in 1997.

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.

Martinez was forced into a car in Beaumont, Calif., in April 1997 as his friends watched. Sixteen days later a forest ranger found the boy’s nude, bound body about 70 miles to the east.

If federal prosecutors fail to win a death sentence in their case, Duncan will be returned to the Idaho state court, where a jury will be impaneled for a death penalty hearing on the murder confessions.