Zachary David O'Neill is due to appear in state district court in Billings for the murder of 18-year-old video store clerk Miranda Fenner and the rape and attempted murder of a second woman during the same time period.
The 39-year-old admitted methamphetamine user pleaded guilty last month under a deal between prosecutors and the defense that calls for a life sentence.
O'Neill confessed to Fenner's slaying as early as 2016 while at a psychiatric hospital in Spokane, Washington. But authorities said he also made false claims at the time that undermined his credibility.
Court documents show investigators continued to pursue other potential suspects until O'Neill again confessed in 2017 and offered details that corroborated his guilt. Fenner's throat was slashed and she was repeatedly stabbed.
O'Neill said he killed Fenner after robbing the video store because he worried that she might be able to identify him.
DNA later matched him to the rape and attempted murder of a newspaper carrier and a second rape of a victim who died in 2013, according to court documents.
"The state believes these three unsolved cases were solved by the defendant's confession and continued cooperation since," Yellowstone County Deputy Chief Attorney Ed Zink wrote in a sentencing memo.
O'Neill told The Associated Press in a jailhouse interview that he was surprised when his April 2016 confession to the killing was met with disbelief.
"I was wondering why they didn't even follow up and investigate," O'Neill said. "They just told me I could go. ... I was like, 'Alright, I tried and I'm done.'"
About a year later, O'Neill walked into the Yellowstone County Detention Center and again claimed responsibility. Detectives said he broke down crying when he confessed.
Because Montana law prohibits convictions based solely on a confession, County Attorney Scott Twito said investigators had to verify O'Neill's claims.
"It's easy to go back in hindsight and connect the dots. It's much harder to do that in the moment and as the years go by. You can't afford to get it wrong," Twito said.
After talking to authorities in 2017, O'Neill headed back to Spokane, where he was arrested for a burglary and a firearms charge. By April of that year, Montana detectives had matched his DNA to the rape case. He was returned to Montana in February 2019 to face charges.
Authorities said the case was complicated by a lack of DNA evidence in Fenner's killing and because numerous others falsely claimed responsibility or were implicated by others.
O'Neill himself was interviewed by detectives early on but suggested others were responsible, said Twito and Yellowstone County Sheriff Mike Linder.
The defendant's O'Neill's former stepfather, David Saylor, said in the days following the crime he told a detective that O'Neill had been at the video store minutes before the killing. "I said: 'If he didn't do it, he damn well knew who did,'" Saylor said.
O'Neill said his step-brother's death in an April 2013 arson fire caused him to feel "shame and regret" and led to his attempts to confess, court documents show. Finally in March 2017, a friend drove O'Neill to the Yellowstone County jail.
He also admitted to the rape and attempted murder of a newspaper carrier and a second rape in the months before Fenner's murder.
Saylor said as a boy his stepson would steal sodas from a convenience store and later cash from at least one business. Over the years the offenses escalated, O'Neill got involved in drugs and he grew increasingly violent.
He lost an eye several years ago after being hit in the head with a baseball bat during a fight and lost most of his teeth from years of smoking methamphetamine.
"He's just a bad person. You just look into his eye and he looked dead," David Saylor said.
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