An attorney for a mentally disabled man serving a 100-year prison term for killing a high school teacher in the Northern Plains' oil patch asked the Montana Supreme Court on Tuesday to vacate his sentence and place him under the custody of state health officials.

The killing of Sherry Arnold in the once-quiet town of Sidney highlighted a significant spike in crime following an oil boom that swept eastern Montana and western North Dakota.

Arnold, 43, was killed when Michael Keith Spell of Parachute, Colorado, and an accomplice attempted to abduct her while she was jogging before dawn near her house in January 2012.

Defense attorney Wendy Holton said in Tuesday's appeal brief that Spell needs to be in a treatment setting such as Montana State Hospital in Warm Springs, not the state prison.

Spell is said to be functionally illiterate with an IQ score of 70, which "falls into the mental retardation range," according to Holton. Keeping him in prison amounts to cruel and unusual punishment in violation of the U.S. and Montana constitutions, she said.

"Michael's disabilities require that he be placed in a facility that has training and experience in dealing with developmentally disabled adults," Holton wrote.

Keeping him in prison puts him at risk of manipulation by other inmates, she added. Spell was convicted by a state judge last year after pleading guilty to murdering the popular math teacher. Co-defendant Lester Van Waters Jr. received 80 years in prison on a charge of deliberate homicide by accountability.

Spell was spared a potential death sentence when state health officials agreed with defense experts who said he was mentally disabled. Defense attorneys also argued that he was incompetent to stand trial, but state District Judge Richard Simonton rejected that claim.

The murder occurred as drilling for oil in the region's Bakken shale formation drew people from around the country who flooded into the rural region for work.

In the days before the crime, Spell and Waters traveled to Sidney from Colorado, looking for work in the oil patch and using crack cocaine throughout their journey.

Waters allegedly told Spell during the trip that crack "brought the devil out in him," and he began talking about kidnapping and killing a woman, according to an affidavit filed by prosecutors.

After the pair spotted Arnold along a Sidney street, Spell tried to grab her, leading to a struggle in which Arnold was choked or otherwise asphyxiated, authorities said.

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