Prosecutors said Monday that a convicted sex offender accused of kidnapping two young children would be charged with first-degree murder in the deaths of three people found slain in the children's home.

Joseph Edward Duncan III (search), 42, already faces kidnapping charges in the abduction of Shasta and Dylan Groene.

He will make a court appearance Tuesday for a probable cause hearing, Kootenai County (search) Prosecutor Bill Douglas said Monday. Douglas did not say when the murder counts would be added.

Brenda Groene, her boyfriend, Mark McKenzie and 13-year-old Slade Groene were killed in their home sometime late on May 15 or early May 16.

Eight-year-old Shasta was spotted July 2 in a Coeur d'Alene (search) restaurant with Duncan, who was arrested.

Dylan, 9, was found dead at a remote campsite in western Montana last week after Shasta gave authorities information about where the children were held.

Kootenai County Sheriff Rocky Watson has said he believes the three people in the house were killed so the perpetrator could kidnap the two children for the purposes of sex. Watson has said the victims were apparently chosen at random, although the killings were carefully planned.