BOISE -- A man accused of strangling a Boise teenager and throwing her body in a creek last September pleaded guilty to first-degree murder Tuesday.

Bruce Marchant, 62, sat calmly in the courtroom as he described killing 18-year-old Sierra Bush, who also went by the name Simon.

"I choked Sierra Bush with my hands, I choked until she died," he said. "She just kept screaming and I didn't know any other way to stop her."

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Additional charges of rape and kidnapping were dropped as part of Marchant's plea deal, although prosecutors will be allowed to use that evidence during their arguments at sentencing.

Marchant, a tenant of Bush's father, said he had met the Boise State freshman "a couple times," but did not know her well.

The attack happened at the home of Bush's father, Marchant said, recounting how he checked to see whether the victim was still alive after he strangled her.

"I put my hand over her mouth to see if she was still breathing," he said. "She wasn't breathing anymore."

Police asked for the public's help finding Bush after her parents reported her missing. Her body was discovered near Idaho City a month later.

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Marchant was arrested at a New York City VA hospital in December after police found the victim's belongings and men's leather gloves stained with her blood in his car.

Tuesday's hearing veered repeatedly into bizarre territory, as Marchant complained about his poor sleep and a lack of sunshine in the jail. He also told the judge he wasn't worried about any fines the court might impose, because he expected to make $100,000 next year - even if he was in prison.

"I have some inventions that I made up, and I actually invented the Toyota Prius, but the Toyota company took off with the idea, and the ran with it. I have a better idea than that," he said. "That would be billions of dollars for that, but I'm working on getting ahold of an invention company."

PREVIOUS: Marchant indicted for murder, rape and kidnapping

Judge Jonathan Medema repeatedly sought assurances that Marchant - who said he had been diagnosed with schizophrenia, PTSD and other mental health issues - understood the hearing and was mentally competent to enter a plea.

At one point, the defendant admitted to being confused, before reversing course and saying he knew what he was agreeing to.

"I'm befuddled. It's just a bad way with me," he said. "It has nothing to do with the court. Maybe I just did too much prison time in places that were bad and guys were getting killed left and right."

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Idaho prisons were not so bad, especially when contrasted with federal lockups, Marchant told the judge. The defendant has spent long stints behind bars for crimes including robbery, arson and shooting at a police officer.

First-degree murder carries a mandatory life sentence, and Marchant said he did not expect he would ever be paroled again. He told Medema he wanted to spare Bush's family the pain of a trial, before hinting his decision to plead had other motivations as well.

"If I go to trial, they'll put the death penalty on me again," he said.

Sentencing is set for Dec. 6.