A plea agreement with prosecutors filed Tuesday calls for a 100-year sentence for Michael Keith Spell, 25, on a charge of deliberate homicide for the 2012 death of Sherry Arnold. A charge of attempted kidnapping would be dropped.
Spell is due to appear Wednesday before state District Judge Richard Simonton in Sidney, a town of 6,200 near the North Dakota border.
Prosecutors want the Parachute, Colorado, man to serve his time in prison. Defence attorney Al Avignone said he will ask for Spell to be sentenced to the state health department, which could allow him to serve his time in a facility for the mentally disabled. They also reserved the right to ask for a lesser sentence.
Simonton will have the final say.
Arnold, a mother of two and popular teacher in close-knit Sidney, was killed just blocks from her house after going out for a morning jog on Jan. 7, 2012.
Her body was found more than two months later, buried in a shallow grave in a once-quiet part of North Dakota that over the past decade has become one of the country's largest oil plays.
Experts for the state and defence have testified that Spell is mildly mentally disabled. He can barely read or write and has a history of low IQ scores. Those findings prompted prosecutors to drop their pursuit of the death penalty.
"He's going to be intellectually disabled until the day he dies," Avignone said. "But that doesn't mean he can't learn and he can't learn life skills."
Richland County attorney Mike Weber said he would oppose any sentence short of prison.
"We'll be arguing for Montana State prison. That's our position," Weber said.
The plea agreement was signed by Spell, his attorneys, Weber and Assistant Attorney General Joel Thompson. Weber said Thompson was instrumental in reaching the agreement.
Accomplice Lester Van Waters pleaded guilty last year in a deal that called for him to testify against Spell if the case went to trial. Waters in turn would be spared the death penalty and receive 100 years in prison.
He has not yet been sentenced.
Court documents say that just before Arnold's murder, Spell and Waters travelled to Sidney from Colorado, using crack cocaine throughout their journey. They told friends and family they were searching for work in the Bakken oil fields along the Montana-North Dakota border.
Authorities have said Spell choked or otherwise asphyxiated Arnold during an attempted abduction.
Spell's attorneys did not dispute that he was involved in the events leading to Arnold's death. Yet they maintained there is no conclusive evidence he was the one who killed her.
Spell implicated Waters as the killer in interviews with law enforcement soon after his arrest.