BOISE - A federal judge says prosecutors can have their own psychologist interview a convicted child killer as they fight to prove he was competent when he waived his right to appeal his death sentence.

But U.S. District Judge Edward Lodge said in a ruling last week that he won't necessarily let them present any of the expert's findings in court - that will be decided down the road.

Joseph Edward Duncan III was sentenced to death in 2008 after admitting he kidnapped and tortured two northern Idaho children before killing one of them. He gave up his appeals, but his former attorneys fought the sentence on his behalf, and last year the 9th U.S. Circuit Court of Appeals ordered Lodge to hold a retroactive competency hearing to determine if Duncan was mentally competent back in 2008 when he waived his rights.

That ruling set into motion a complicated legal process with high stakes: Duncan, though convicted of five murders spanning three states in three separate courts, has only been sentenced to death in Idaho's federal case, and that sentence is now in jeopardy.

During his 2008 sentencing hearing, federal prosecutors said Duncan snatched Dylan Groene and his 8-year-old sister from their North Idaho home on a spring day in 2005 after killing their older brother, mother and mother's fiance. Duncan kept the children at a remote Montana campsite for weeks before killing Dylan and returning with Dylan's sister to Coeur d'Alene, where he was arrested.

After Duncan was given three death sentences in Idaho's federal court for Dylan's murder and other federal crimes, prosecutors in North Idaho's Kootenai County opted not to seek the death penalty for the murders of Slade and Brenda Groene and Mark McKenzie, and a state court judge sentenced Duncan to life in prison.

Likewise, in a subsequent trial for the 1997 murder of 11-year-old Anthony Martinez in Riverside County, Calif. - which Duncan confessed after his arrest in Idaho - the local district attorney opted not to seek the death penalty after talking with Martinez' family and noting that Duncan already faced death three times over from Idaho's federal court case.

John Hall, a spokesman for the District Attorney's office in Riverside County, Calif., said Monday they were confident that Duncan's death penalty would be upheld.

The Riverside County prosecutors are working with the U.S. Attorney's Office in Idaho for the competency hearing, he said.

The case will hinge on Duncan's state of mind about four years ago. When Duncan first decided to represent himself in 2008, his attorneys claimed he wasn't competent. The judge, Lodge, had him examined by a local psychologist who found that while Duncan's thoughts were "somewhat unusual," Duncan wasn't delusional and that he'd waived his right to an attorney knowingly and voluntarily.

But Duncan's attorneys have argued that despite sometimes giving the appearance of being capable of rational thought, Duncan is mentally ill and didn't have the capacity to make a rational decision about whether to waive his appeals.

"Like most delusional individuals, Mr. Duncan's capacity for rational thought varies across different decision-making domains," his attorney Michael Burt wrote in a court document filed late last week.

If Lodge finds that Duncan wasn't competent when he gave up his appeals, the judge will have to assess whether he was competent during his sentencing hearing. That could lead to a new sentencing hearing, or a new sentence of life in prison.

"In this particular case, as the Court is well aware, the Court's decision may very well determine whether Mr. Duncan lives or dies," Burt wrote.

U.S. Attorney Wendy Olson has pointed out in court filings that established case law clearly shows that a pattern of consistent, rational behavior, thinking and decision-making over a long period of time tends to show at a defendant's decision-making process at any one time was rational.

Duncan was examined by several experts in the months before and after his decision to waive his appeal, Olson noted, and the courts' own expert as well as a California court found Duncan to be competent.

The competency hearing is set for January 2013.