BILLINGS, Mont. (AP) — More than 17 years after he slashed the throat of a teenage clerk while robbing a Montana video store, Zachary O’Neill says guilt overcame him and he tried to tell authorities.

But it was another three years after he first confessed in a psychiatric hospital, and later to sheriff’s investigators, before the admitted drug addict and petty criminal was prosecuted for the 1998 murder of 18-year-old Miranda Fenner.

“I was wondering why they didn’t even follow up and investigate,” O’Neill said about his April 2016 confession at a psychiatric hospital in Spokane, Washington.

“They just told me I could go,” O’Neill told The Associated Press in a jailhouse interview. “I was like, ‘Alright, I tried and I’m done.’”

On Friday, O’Neill, 39, is expected to be sentenced to life in prison after pleading guilty last month to Fenner’s murder and the attempted killing and rape of another victim during the same time period.

His thwarted confessions underscore a series of apparent missed opportunities for investigators.

Even after he walked into the Yellowstone County jail in 2017 and again claimed responsibility, he was not immediately arrested as skeptical authorities said they needed to check out his story. Detectives, who said they had little usable DNA evidence from the Fenner killing, took his DNA, released O’Neill and set out to confirm his story.

Meanwhile, O’Neill kept trying to confess, approaching the Spokane police two to three times in subsequent weeks until they “told him to please stop coming in and confessing,” according to his sister, Natanya O’Neill.

Yellowstone County Attorney Scott Twito confirmed the hospital confession, but said it did not seem credible: O’Neill claimed to have also killed another person, who was still alive.

“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.

Twito said he was not aware of the confessions Natanya O’Neill claims. Spokane police did not immediately answer questions about the confessions.

When he spoke with Yellowstone County investigators in 2017, O’Neill said he “didn’t care too much” at first about what he had done to Fenner. He said he was stealing to support his drug use and had smoked meth before renting several movies that night. After his mother learned one of the movies was pornographic she sent him back. He said he decided to rob the store and killed Fenner so she could not identify him.

He was able to describe Fenner’s clothing and other details about the crime that were not released publicly, court records 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 1998 rape, where the victim was a newspaper carrier. O’Neill also slashed the victim’s throat in that case, but she survived.

He was returned to Montana in February 2019 to face charges.

Authorities said the case was complicated not only by a lack of DNA evidence in the killing, but because numerous others falsely claimed responsibility or were implicated by others. O’Neill himself was interviewed by detectives early on but he pointed to others, said Twito and Yellowstone County Sheriff Mike Linder.

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 Fenner’s killing. “I said: ‘If he didn’t do it, he damn well knew who did,’” Saylor said.