BILLINGS (AP) — A defense attorney says a mentally-disabled Colorado man serving 100 years in prison for killing a Montana teacher should be transferred to the custody of state mental health officials.

Attorney Wendy Holton said in arguments filed Tuesday with the state Supreme Court that Michael Keith Spell needs treatment he won't receive in prison.

Spell's 2012 killing of Sherry Arnold in the once-quiet town of Sidney highlighted a significant spike in crime following an oil boom that swept the Northern Plains.

He was convicted last year of murdering the popular math teacher after attempting to abduct her as she was jogging near her home in 2012. An accomplice received 80 years in prison.

Spell was spared a potential death sentence when state health officials agreed that he was mentally disabled.