Joseph Duncan: Gruesome Idaho Serial Killer, Dead at 58


Joseph Edward Duncan, was an American convicted serial killer and child molester. Responsible for the deaths of Sammiejo White, age 11; White’s sister, Carmen Cubias, aged 9; Anothony Martinez, age 10; Brenda Groene, age 40; her boyfriend, Mark Mckenzie, aged 37; Brenda’s son, Joseph Duncan, age 13; and Brenda’s other son, Dylan Groene, age 9. 

Born in Tacoma, Washington, Joseph Duncan has had a criminal record since he was 15 years old. At the age of 15 (1978), Duncan was sentenced to 20 years in prison after sexually assaulting a boy. During his years of being a serial killer, Duncan had killed in the states of California, Washington, and Idaho. Duncan was charged with 5 counts of murder, 1 count of auto theft, 4 counts of kidnapping, 1 count of sodomy, 1 count of drug possesion, and 1 count of parole violation. His penalties were; by state (Idaho and California) Life imprisonment; by Federal, death. He was sentenced to death in 2008 after evidence came out, convicting Duncan of the murder of 10 year old, Anothy Martinez. 

The Groene family’s attack (which happened in 2004-2005) was during Joseph Duncan’s run from Child Molestation charges in Minnesota. His method of murder was hitting the heads of his victims with a hammer. The bodies of Brenda, Joseph Duncan, and Mark Mckenzie, were found in Kootenai County, Idaho. The two younger children Dylan and Shasta Groene, were kidnapped, Dylan was later murdered by Duncan and his body was found in Montana. According to Joseph Duncan, he didn’t kill Shasta because she “taught him how to love.”  The case of Anthony Martinez’s death was initially a cold case until Duncan’s trial in 2005. During this case he confessed to Sammiejo White and Carmen Cubias’s murders, unfortunately, Duncan wasn’t convicted of those murders. 

In March of 2021, Joseph Dunca died in Indiana Hospital after suffering from terminal brain cancer and stage 4 brain cancer. According to, “He underwent brain surgery in October 2020, after being diagnosed with Glioblastoma- stage 4 brain cancer.” It has been reported that Duncan refused any sort of treatment, and the doctors said that Duncan would only have 6 months left to live. 

Victim Shasta Groene spoke out on the matter of Duncan’s death saying, “I woke up feeling like my soul was finally free.”