Police on Friday night positively identified the victim as Ethan Anderson, who was listed as missing.
The burned bodies of the boy and his mother, 44-year-old Christina Anderson, were discovered last Sunday in a log-style cabin in Boulevard, Calif., just east of San Diego last Sunday.
The burned-out home belonged to 40-year-old James Lee DiMaggio, who authorities believe abducted the woman's daughter, 16-year-old Hanna Anderson.
Investigators had to identify Ethan's body through DNA extraction from his bone marrow.
Authorities are now focusing on Idaho in their search for the girl and the man suspected of killing the teen's mother and brother.
A horseback rider had reported seeing two people matching the pair's descriptions in the Cascade area northeast of Boise.
On Friday morning, a car belonging to DiMaggio was found covered in brush in the backcountry of Idaho's Valley County.
Police are worried the teen and DiMaggio are in what has been described as the largest roadless area in the lower 48 states.
The search area is bisected by a waterway that winds through steep canyons and dense forests.
Away from the river, it's easy to disappear, said Jared Hopkinson, the owner of Rocky Mountain River Tours in Stanley, Idaho.
Hopkinson said a backpacker can hike for days without seeing another person.
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