I believe that marys abusive bf did it,the sherriff wanted richard to be guilty cos when he was younger and richards mum died in the house fire,the sherriff then suspected richard guilty of her and her fellas deaths,he wanted to get him on something,anything,so concluded that richard killed mary when in fact marys bf,jealous over her friendship with richard,strangled her and dumped her body in the river,because the sherriff had spied on richard and mary the night before and witnessed an argument between the couple(who wanted to be together)he knew he could accuse him!!