SIDNEY, Mont. » A man was sentenced to 100 years in prison Friday in a Montana teacher’s murder — a case that local officials say robbed the victim’s rural hometown of its innocence amid an oil boom that’s swept across the Northern Plains.

Michael Keith Spell, 25, of Parachute, Colorado, was sentenced following a hearing before state District Judge Richard Simonton in Sidney.

Spell, who is mentally disabled and described by the defense as functionally illiterate, pleaded guilty in October to killing Sherry Arnold, 43, after attempting to abduct the popular math teacher as she was jogging near her home in 2012.

Arnold’s body was found months later buried in a shallow grave in a rural area near Williston, North Dakota.

Co-defendant Lester Van Waters Jr. was sentenced in December to 80 years in prison on a charge of deliberate homicide by accountability.

Spell’s defense attorneys portrayed Waters as the one who hatched the pair’s plan to kidnap a random woman for sex. They alleged Waters threatened to kill Spell unless he went along.

But Simonton noted Spell did not try to escape from Waters when he had the chance, including in the final moments between leaving Waters’ car and grabbing Arnold from the side of the road.

Spell, wearing a cardigan and checkered shirt, kept his hands in his pockets and head bowed as the sentence was pronounced. He expressed remorse in a brief statement to the court.

"I know that I hurt a lot of people. I’m just hoping that someday they’ll be able to forgive me," he said. He will become eligible for parole in 2037.

Defense attorneys had asked that Spell be placed into state health officials’ custody for placement in an institution for the mentally disabled. They said Spell’s mental problems made him unable to conform to the law.

In the days before Arnold’s killing, Spell and Waters traveled to Sidney from Colorado, looking for work in the booming Bakken 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.

The victim’s husband, Gary Arnold, said he was relieved the case had come to a resolution after causing so much anguish for his family and the community.

"I’m even more relieved the judge imposed the full sentence," an emotional Arnold said.

The murder occurred as drilling for oil in the Bakken shale formation beneath eastern Montana and western North Dakota was getting into full swing, with people from around the country flooding into the region for work.

The boom has almost doubled Sidney’s population to an estimated 9,300 people, bringing with it more traffic and higher housing costs.

Arnold’s death brought another kind of change: Right away, people started locking their front doors and quit leaving their keys in their vehicles. Many in the community remain reluctant to venture out alone, according to local residents and city officials.

"The innocence of Sidney has definitely changed," Sidney Mayor Rick Norby said. "You’re never going to get over it."

Spell was spared a potential death sentence after state health officials agreed with the defense that he was mentally disabled. But Simonton in May rejected arguments from the defense that Spell’s history of low IQ scores, his difficulty reading and other mental problems rendered him incompetent to stand trial.

Psychologist Craig Beaver, a defense witness, said Spell has a child’s mentality and a subservient personality that allowed him to be manipulated by Waters.

But prosecutor and Richland County Attorney Mike Weber noted that Spell acted on his own volition when he ingested crack cocaine, alcohol and marijuana leading up to the attack.

Defense attorneys Al Avignone and Lisa Banick said Friday’s sentence will be appealed based on their continued belief that Spell was not fit for trial.