The two were found dead by a realtor in their home on Deming Road on Oct. 12, 2017.
At the time, police said they believed 86-year-old Margaret Montgomery Gibson was killed by her son, 58-year-old Eric Dana Gibson.
According to the autopsy, Eric was found with a knife "clenched in his hand" and he was "in a prone position overlying his mother" in the bathroom.
Two additional knives were found nearby.
The autopsy determined that Eric died from "hypertensive and atherosclerotic cardiovascular disease" with obesity listed as a contributing factor. His manner of death was ruled natural.
According to the autopsy, Eric had a history of schizophrenia.
Margaret's autopsy showed that she had at least 74 sharp force injuries to the head, neck, torso and left hand. Her manner of death was ruled a homicide.
According to the autopsy report, Eric was unhappy that his mother planned to move both of them to California.