sherry arnold , a 44-year-old math teacher from sidney , montana , was on a morning jog on january 7 , 2012 , when she was abducted .
michael keith spell , 25 , of parachute , colorado , confessed at the time to pulling her into a car while his friend , lester van walters jr , strangled her .
arnold was then buried 50 miles away near williston , north dakota .
defense experts said spell did not understand the case against and was mentally unfit to stand trail .
prosecutors agreed he was mentally ill but said he could stand trial , recommending 100 years prison .