A student had a list of 33 names of students and staff members from his Southern California high school who he wanted to shoot, and a detailed plan to do it, authorities said Wednesday.
The student at Encore High School had the intent and capability to carry out the attack, San Bernardino County Sheriff John McMahon said at a briefing for parents and press.
"There certainly was the potential in this case for harm," McMahon said. "That plot was very well-detailed and he was very set on doing something at Encore High School."
McMahon said the boy, now being held in a juvenile jail, had "limited access" to firearms, but did not elaborate.
The investigation began when a parent on Saturday told the Sheriff's Department about threats on social media. After detectives talked to the student on Sunday, and to school staff and students on Monday, they obtained a search warrant for both of the boys' parents' homes.
That search turned up evidence, including the plans and list of 33 names and, that led to the boy's arrest the same day.
McMahon said explosives were mentioned in the plan, but searches including a combing of the high school by dogs didn't reveal any.
Encore is a charter high school in Hesperia, a Mojave Desert City about 80 miles northeast of Los Angeles.
This story has been updated.