Investigators discovered Willson and his mother were the only people in the home Thursday night, when they reportedly argued about keeping a dog the teenager had brought home with him a few weeks prior.
Lisa allegedly refused and told Andrew the pet would instead be kept with his father in Dansville, about 11 miles away, Detective Charles Buckland said.
Willson told detectives he took a .22 Magnum rifle from a locked cabinet and shot his mother in the back of the head, before tossing the rifle nearby, Buckland said.
