A man who already pleaded guilty to killing three members of an Idaho family in a 2005 attack has been charged in federal court with kidnapping the family's two youngest children and killing one of them.

The indictment against Joseph Edward Duncan III, issued Thursday by a federal grand jury in Coeur d'Alene, will allow the government to seek the death penalty, U.S. Attorney Tom Moss said.

It accuses Duncan 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's lawyer, Roger Peven, said his client would plead not guilty at a scheduled court appearance Friday.

"This will get the process going," Peven said. "We've been anticipating it for quite some time."

Shasta Groene was rescued as she and Duncan ate at a Coeur d'Alene, Idaho, restaurant in July 2005, about seven weeks after the family was attacked. Duncan has been quoted in court documents as saying he was trying to return the girl to her father.

Among the charges against Duncan, 43, are kidnapping resulting in death, sexual abuse of both children and firearms counts.

The grand jury alleged Duncan killed Dylan in an "especially heinous, cruel, and depraved manner," according to federal prosecutors. "The grand jury also found that the child's killing involved torture and serious physical abuse."

Shasta has been living with her father, Steve Groene, who did not immediately respond to an e-mail request for comment on the federal charges. He is unable to speak because of throat cancer surgery last year.

Duncan was also charged Thursday in a California state court in the 1997 abduction and slaying of 10-year-old Anthony Martinez, whose bound, nude body was buried under a rock pile in the desert.

He is also considered the prime suspect in the slayings of two children near Seattle.

Peven told The Associated Press the federal case will be resolved before any additional cases are tried in state court.

On Tuesday, federal prosecutors in Idaho charged Duncan with driving a stolen vehicle across state lines. The charge was considered a placeholder to make sure Duncan was not extradited for crimes in other states before the federal case was completed.

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.

Duncan stalked the Groene family for several days, then entered the home and bound and fatally bludgeoned the two adults and the teen.

A state judge sentenced Duncan to life in prison without parole for the kidnappings, but sentencing on the murder counts was deferred while the federal government prepared its charges.

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