Ethan Anderson’s body was only identified after investigators carried out tests on DNA extracted from bone marrow taken from his charred body in the house in San Diego.
The body was found in a garage close to a room where the dead body of his 44-year-old mother, Christina Anderson, was discovered. Police said she was murdered but have refused to say how she was killed.
Ethan’s 16-year-old sister, Hannah Anderson, remains missing along with her alleged kidnapper, James DiMaggio, who is reported to have had “a crush” on the teenager.
The dead bodies were found in DiMaggio’s house in San Diego, which police believe he torched a week ago before taking off with Hannah.
The US authorities have issued child-abduction alerts across the west of the country and have also asked authorities in Canada and Mexico to watch for the 40-year-old fugitive and Hannah.
Police revealed there was a possible sighting of DiMaggio’s vehicle, a blue Nissan Versa, near Alturas in north-east California, and another along the same highway in Lakeview, Oregon.
A school friend of Hannah’s, 15-year-old Marissa Chavez, has told how DiMaggio had developed a crush on Hannah and told her he would date her if they were the same age.
She told the San Diego Union-Tribune newspaper: “He said he had a crush on her, but didn’t mean it in an intimate way. He said ‘If I was your age, I’d date you.’”
The friend described how Hannah asked her to join them when DiMaggio, a family friend, picked her up because she was uncomfortable with the attention.
Brett Anderson, Christina Anderson’s husband and father to Hannah and Ethan, has pleaded with DiMaggio, a computer technician, to return his daughter safely.
He said: “Jim, I can’t fathom what you were thinking. The damage is done.” And addressing his daughter, he said: “If you have a chance, you take it. You run. You’ll be found.”
Anderson joined hundreds of family friends and neighbours at a candlelight vigil in the parking lot of El Capitan High School in Lakeside, San Diego, where Hannah was about to start her junior year.
Pink paper cups stuffed between holes in the fence read “Pray Hannah” and dozens of white, blue and pink balloons lifted into the air.
Hannah and Ethan’s grandmother, Sara Britt, also attended a candlelight vigil in Lakeside to pray for the safe return of the teenager. She spent time chatting to friends of her grandchildren, including distraught 11-year-old Christian “Boogie” Ingram, who was Ethan’s best friend.
Police have been scouring the Idaho outback looking for clues to the pair’s where-abouts.
Chris Grove, district ranger with the Salmon-Challis National Forest, which oversees the area, said his team was checking for any evidence the pair had been sleeping in the forest in recent days.
He said: “Nighttime temperatures in the high country are already dropping below freezing, and cross-country travellers will face numerous obstacles on trails such as large, fallen trees and steep grades that may lead to nowhere.”
Police also revealed that horseback riders returning from Idaho’s wilderness have told them they may have encountered DiMaggio and Hannah.
The group said they spoke briefly with the man and his young companion, and only realised after returning and seeing news reports that they may have met the pair.
DiMaggio’s car, which was covered in tree branches and had its licence plates removed, was found in the same area on Friday.
Police feared the car could be rigged with explosives, but a bomb squad found none in or near the vehicle.
Captain Duncan Fraser, of San Diego County Sheriff’s Department, said that the camping gear being used by DiMaggio and the girl had been purchased several weeks earlier.
“We believe this was a planned event,” he said.
Thousands of messages of support have been written on Hannah’s Facebook page.
“You would never see her without a smile on her face,” said Marlee Friszell, 16, who attended Hannah’s birthday party last week.
San Diego County Sheriff Bill Gore said he is confident that Hannah will be returned safely.
“We hope that law enforcement will find them up in the mountains there and return them safely,” he said.
Gore said it did not appear that Hannah was being held against her will.