Convicted killer Joseph Duncan has terminal brain cancer, which will likely kill him before his federal death sentence is carried out, court documents say, according to reports this week. 

Duncan, 58, a native of Tacoma, Wash., has been on federal death row in Indiana for years as his appeals move forward after he was convicted of killing four Idaho family members in 2005.

He underwent brain surgery last October and was diagnosed with glioblastoma, stage four brain cancer, according to court records. The data shows he has declined chemotherapy and radiation, KXLY-TV of Spokane, Wash., reported.

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Joseph Edward Duncan III appears at his video arraignment from the Kootenai County jail on two counts of kidnapping in the first degree on July 5, 2005, in Coeur d' Alene, Idaho. (Getty Images)

"In November 2020, Bureau of Prisons medical staff determined his life expectancy to be 6-12 months ... medical staff discussed end-of-life preparations with Mr. Duncan," court documents stated. 

Duncan was convicted of killing Brenda Groene, Slade Groene, Dylan Groene, and Mark McKenzie -- four members of a Coeur d’Alene, Idaho family -- roughly 16 years ago. He kidnapped the family's two children, Dylan and Shasta, from their home and tortured them in Montana before killing the boy.

Shasta was the only survivor of the rampage. She was rescued after Duncan stopped at a restaurant in Coeur d'Alene and the girl was recognized by staff.

Duncan, a convicted sex offender, was also linked by DNA to the murder of a California boy, 10. He was convicted of that crime after he was convicted of the Idaho murders and kidnappings.

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During court testimony in 2008, Spokane County Sheriff Deputy Dale Moyer said he left enforcement for several months to reflect on what he had seen at the Groene home, according to KXLY-TV.

"This case pushed me to the end," Moyer said.

According to a medical report dated last November, a doctor spoke about how Duncan needed help cleaning his cell "once or twice a week" due to his health ailments. 

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"It might be a little bit much for him to consistently bend over and clean the area considering his balance issues," the report stated, according to the station

Duncan's health development comes as Joe Biden -- the first sitting U.S. president to openly oppose the death penalty -- discussed the possibility of instructing the Department of Justice to stop scheduling new executions, officials told AP last month. 

The Associated Press contributed to this report.