Christina Anderson, 44, was found with her 8-year-old son, both dead, in the burned remains of James DiMaggio's house in Boulevard, about San Diego miles east of San Diego.
A San Diego County Medical Examiner's autopsy report released Monday said she died from at least a dozen blows to the head. Anderson was found with duct tape wrapped around her neck and mouth and her ankles tied with plastic cable.
Son Ethan Anderson, 8, was also found in the rubble of the house. His remains were so badly charred that his cause of death could not be determined, according to investigators.
James DiMaggio, 40, was killed by FBI agents in the Idaho wilderness Aug. 10, one week after he allegedly abducted 16-year-old Hannah Anderson and killed Christina and Ethan Anderson at his home in Boulevard, 65 miles east of San Diego. Hannah Anderson was rescued and returned safely to California.
Christina Anderson asked DiMaggio to take Hannah to cheerleading camp Aug. 3 because she was at Ethan's football practice and unable to take her, San Diego County Sheriff's Detective Troy DuGal said in the autopsy report.
Hannah's disappearance triggered a massive search spanning much of the western United States and parts of Canada and Mexico. Authorities have said DiMaggio set fire to his home with a timer, giving him a 20-hour jump on them.
The autopsy report says Christina Anderson suffered blows to the back of her head, forehead and nose but doesn't describe which instruments were used. Authorities have said firefighters found a crowbar near her body.
The Associated Press contributed to this report.