BILLINGS, Mont. » A mentally disabled Colorado man was scheduled to be sentenced Friday for the murder of a Montana teacher whose death highlighted the downsides of an oil boom that has brought thousands of newcomers to the remote Northern Plains.

Prosecutors recommended 100 years in prison for Michael Keith Spell, 25, of Parachute, Colorado, after he pleaded guilty in the 2012 killing of Sherry Arnold. The popular Sidney High School math teacher was choked or otherwise asphyxiated after Spell tried to abduct her while she was jogging.

Defense attorneys want Spell sentenced to the custody of state health officials so he can be placed in an institution for the mentally disabled. They allege that co-defendant Lester Van Waters, 51, had orchestrated a plan to kidnap someone and threatened to kill Spell with a utility razor unless he went along.

Arnold’s body was found months after her disappearance in a shallow grave in a rural area near Williston, North Dakota.

Spell said during a change-of-plea hearing in October that he was unsure if he or Waters had killed Arnold.

The defendants had come to the region to work in the Bakken oil patch, where an oil boom has transformed prairie towns like Sidney into bustling centers of industry.

After Arnold’s murder, the case emerged as a testament to the social upheaval caused by the boom. Authorities say crime has spiked in the Bakken as traffickers of methamphetamine and other drugs seek to profit off oil field workers flush with cash.

The murder case is before state District Judge Richard Simonton. He sided with Richland County prosecutors in ruling last year that Spell was fit to stand trial. The judge rejected defense arguments that Spell’s low IQ scores and history of mental problems made him incompetent.

Defense attorneys Al Avignone and Lisa Banick said they planned to appeal Simonton’s fitness ruling no matter the outcome of Friday’s sentencing hearing.