A 48-year-old man was charged with manslaughter after he told deputies he accidentally shot a 73-year-old woman to death at a home in Valrico. Now, the Hillsborough County Sheriff's Office has upgraded his charge to premeditated first-degree murder. Michael Zammiello is accused of shooting 73-year-old Shirley St. Pearre to death at a home on Amber Ridge Lane in Valrico, the agency said in a news release. Deputies reportedly learned of the incident Wednesday afternoon after receiving a call from Zammiello regarding an "accidental injury." When they arrived at the home, they found St. Pearre with a gunshot wound to her head. She was declared dead after being taken to a nearby hospital. When questioned, Zammiello reportedly admitted that he accidentally shot the 73-year-old woman inside the home. He was arrested without incident and charged with one count of manslaughter with a weapon. But, following an autopsy conducted by the Hillsborough County Medical Examiner's Office, deputies upgraded his charge to premeditated first-degree murder with a firearm. In an updated news release, Hillsborough County Sheriff Chad Chronister said the evidence uncovered in the autopsy "leaves no doubt" regarding Zammiello's intent. "This was a senseless act of violence, and our detectives have worked tirelessly to ensure justice is served," Chronister stated. The investigation is ongoing.