(AP) — A Colorado man is set to plead guilty to killing a small-town Montana high school teacher in a slaying that shone a harsh light on the darker side of the Northern Plains' oil boom. 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. Defense 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. Experts for the state and defense have testified that Spell is mildly mentally disabled. The plea agreement was signed by Spell, his attorneys, Weber and Assistant Attorney General Joel Thompson.