Investigators said they did not yet know what led the student to open fire at the school 40 miles (65 km) north of Los Angeles.
Police said the accused shooter had acted alone. Investigators descended on his family home, blocking off the street. They found no further danger there.
A next-door neighbor, registered nurse Jared Axen, said the suspect had seemed introverted, quiet and sad, possibly despondent over the loss of his father from a heart attack in December 2017.
Axen, 33, said it was the boy who found his father deceased, not long after the older man had regained his sobriety and gotten his life “back on track” after years of struggling with alcohol abuse.
“I would say he (the boy) was hurting and couldn’t ask for help,” Axen said of the suspect, who was a track athlete at the school, involved in Boy Scouts and liked the outdoors, going on hunting trips with his father.
He was of mixed race, born to Japanese-born mother and white father, with an older sister who became a nurse and moved away.
“I would ask him how school was … he would never bring up concerns of bullying or being a loaner there,” Axen said.