BOISE, Idaho (AP) - Convicted child-killer Joseph Duncan, already sentenced to die, should be given three life prison terms when he's sentenced Monday on additional federal convictions, a prosecutor says.

Duncan has been given three death sentences in U.S. District Court for the 2005 kidnapping, sexual exploitation and murder of 9-year-old Dylan Groene, of Coeur d'Alene.

On Monday, he is to be sentenced on seven other counts to which he's pleaded guilty, including the kidnapping and sexual assault of Dylan's then-8-year-old sister, Shasta, as well as firearms charges and transportation of a stolen vehicle across state lines.

"The criminal conduct that the defendant engaged in against these children is perhaps the most horrific, cruel and depraved criminal episode in Idaho's history," U.S. Attorney Tom Moss argued in court documents obtained by The Spokesman-Review. "Consecutive life sentences … would help ensure that this defendant, even if his death sentence were somehow overturned should he elect to appeal, is never allowed access to the public."

Duncan lived in Fargo, N.D., before his arrest.

In May 2005, the bodies of 13-year-old Slade Groene, his mother, Brenda Groene, and her fiance, Mark McKenzie, were discovered in their Coeur d'Alene home. The two youngest children in the home - Shasta and Dylan - were missing.

Duncan, 45, a convicted pedophile originally from Tacoma, Wash., had killed the family so he could kidnap the two children. He took them to a remote Montana campsite where he tortured and abused them for weeks, videotaping some of the abuse. He shot Dylan Groene to death at the campsite, and a few days later took Shasta back to Coeur d'Alene. The girl was rescued after a waitress recognized her while the two were at a restaurant.

Duncan pleaded guilty to the killings at the house in state court in 2006, but the federal court handled the charges related to the kidnapping, abuse and Dylan's murder. In December 2007, he pleaded guilty to all 10 federal counts, three of which carried the death penalty.

He was sentenced to death in August following a penalty hearing in which jurors heard harrowing testimony of Duncan's torture of the children.

At Monday's sentencing, Moss wants Duncan to receive consecutive life sentences for the aggravated sexual assaults of the two children, and a life sentence to run concurrently for kidnapping Shasta.

"A single life sentence is not sufficient to address the nature, circumstances and seriousness of these offenses," Moss wrote. The assaults of the children "independently warrant a sentence that insures he will never again be able to commit a crime against children."

Moss also is seeking concurrent 10-year sentences on each of the four other charges: three relating to a shotgun Duncan stole and used in the crimes, and one for the theft of a vehicle from a rental company.

Duncan, who is representing himself, did not submit any arguments concerning the sentencing.

Duncan spent most of his life behind bars following his 1980 conviction for raping a 14-year-old boy in Tacoma at the point of a stolen gun. Each time he got out - for several years in the mid-1990s on parole that he eventually violated, and finally in 2000 at the end of his 20-year sentence - prosecutors say his crimes resumed.

Federal prosecutors say Duncan has acknowledged killing Carmen Cubias, 9, and Sammiejo White, 11, in Seattle in 1996 and Anthony Martinez, 10, in Riverside County, Calif., in 1997.

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Information from: The Spokesman-Review, http://www.spokesmanreview.com