Who was Joseph Duncan III and how did he die? Details explored ahead of People Magazine Investigates episode

The new season of People Magazine Investigates will kick off on June 6, 2022, with the gruesome massacre of the Groene family at the hands of Joseph Duncan III, a serial killer and registered sex offender, in the small town of Idaho. The murderer, Joseph Duncan III, allegedly killed Brenda Groene, her boyfriend Mark McKenzie, and the eldest son Slade Groene with a hammer, before kidnapping 9-year-old Dylan Groene and his 8-year-old sister Shasta Groene, who is the sole survivor of the massacre.

After kidnapping the two young children, he tortured them for weeks and ended up killing Dylan Groene before he was spotted with the 8-year-old Shasta at Denny’s restaurant in Coeur d’Alene in July 2005. He was sentenced to death soon after he was apprehended for three counts of first-degree murder. He died of stage IV brain cancer while in custody at Terra Haute Federal Correctional Institution in 2021.

Continue reading for more information on the heinous crime and the perpetrator Joseph Duncan III.

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Who was Joseph Duncan III and what did he do?

Joseph Duncan III was a registered sex offender who confessed to assaulting as many as 13 young boys by the time he was 16. Though he had his share of other crimes, including assault and another murder that was not linked to him till his arrest, his most brutal crime was the massacre of the Groene family in May 2005.

Duncan allegedly spotted two Groene children playing in the yard while passing by the Idaho Panhandle on Interstate 90. He reportedly staked out their home for days, later using night vision goggles to break into the Groene's home.

He allegedly tied up Brenda Groene, 40, Mark McKenzie, 37, and Slade Groene, 13, before beating them to death with a hammer. He then proceeded to kidnap Shasta, 8, and Dylan, 9. He allegedly took them to the wilderness of western Montana, skipping campsite to campsite to avoid the law enforcement.

He assaulted both the children and killed the young boy. He was spotted with the missing Shasta in a restaurant in Coeur d'Alene, where employees and customers identified the girl and notified the police. He was arrested in July 2005. Dylan's body was later found at one of the campsites.

Acting U.S. Attorney Rafael M. Gonzalez, Jr. said:

"This crime was horrendous and its impact on the families, the community, the jurors, court staff, our litigation team, and law enforcement were far reaching...While his death will not bring back the lives cut so tragically short or remove the indelible memories of his unspeakable acts, perhaps death will now allow space for some degree of healing, peace and closure."

Joseph Duncan III pleaded guilty to the murders in court in 2006. He also pleaded guilty to his violent acts against the children in 2007. Investigators also recovered photographic and videographic evidence of Duncan's crimes.

In 2011, Joesph Duncan III also pleaded guilty to the 1997 murder of a 10-year-old boy, Anthony Martinez.

Joseph Duncan III died of terminal brain cancer on March 28, 2022, while in custody on death row at Terra Haute Federal Correctional Institution. He was 58 at the time.

Shasta Groene, the sole survivor of the massacre, now in her mid-20s, released a statement saying:

"One thing is for sure, he does not exist anymore. Now, we can live our lives knowing that. For so long I have been struggling with hate towards that man. Today, I woke up feeling like my soul was finally free."

Investigation Discovery's People Magazine Investigates will cover the details of the event in its upcoming episode titled Groene Family Massacre.

People Magazine Investigates will air at 9.00 PM ET on June 6, 2022.