California Highway Patrol Officer Stefanie Buck said investigators have not yet been able to interview the car's lone survivor to determine why sixth-grader Jose Manuel Silva Covarrubias was driving.
"For whatever reason the 11-year-old didn't stop at the stop sign, and drove the Honda into the path of the Ford," Buck said. "It's a tragedy any way you look at it."
His sister, Elizabeth Silva Covarrubias, 6, and their mother, Maria Covarrubias, 30, both of Woodville, were also killed in the accident. Neither the mother nor daughter was wearing a seatbelt, and both were ejected from the back seat.
A family friend, Froylon Gonzalez, was sent to a Fresno hospital with life-threatening injuries. Gonzalez, 34, of Woodville, was wearing his seat belt in the passenger seat when the crash happened at a rural intersection.
Felipe Zamora Gonzalez, the man's brother, said the family was returning from Jose's soccer team practice, and added that he had never seen Jose drive before.
Investigators said the boy was driving a 1993 Honda when he ran a stop sign and was hit broadside. The driver of that vehicle, Charles Seguin, 44, of Porterville, sustained minor injuries.
It does not appear drugs or alcohol were a factor in the crash, nor did the winter fog typical of the area obscure the view, Buck said.
The accident happened at a rural intersection outside Porterville, which is about 70 miles southeast of Fresno.
Dondreia Bradley, the superintendent of Woodville Union School District said she was "stunned" to hear that Jose had been behind the wheel.
He came from "a wonderful family" with a supportive mother, she said.