Convicted murderer Joseph Duncan is back in a federal court in Idaho.

Duncan was sentenced to death in 2008 after pleading guilty to kidnapping and torturing two northern Idaho children before killing one of them in western Montana.

He is expected to return to U.S. District Court in Boise Tuesday for the first of several days of testimony to determine if he was mentally competent when he waived his right to appeal his sentence. Duncan's former attorneys argue he wasn't mentally competent to waive the appeal and are fighting his sentence on his behalf.

Duncan has been convicted of five different murders in Idaho, Montana and California. But this competency hearing focuses only on the crimes he committed against young Dylan Groene and his 8-year-old sister, Shasta, in 2005.