Robert Jacobsen, general counsel for Norwalk-La Mirada Unified School District, told the Whittier Daily News that the school safety officer heard the student threaten to bring a gun when he returned to the school.
Sheriff's officials said a campus security officer overheard a disgruntled student threatening a school shooting.
Sheriff's spokeswoman Nicole Nishida tells The Associated Press that deputies discovered "multiple guns and ammunition" after searching the student's home.
One of the AR-15s was registered to the student's 28-year-old brother, a military veteran, but the other rifle was unregistered, which is a crime in California.
Southern California authorities say they took action immediately on a student's threat to open fire at his high school even though he claimed it was a joke.More news: Apple may launch iPhone SE 2 at WWDC in June
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Chavez says the student said he was kidding and didn't mean it.
The teenager was arrested for making criminal threats while the older brother was arrested on charges including possession of an assault weapon and failure to register a handgun, McDonnell said. But the guard informed administrators, who brought in the Los Angeles County Sheriff's Department.
The sheriff said that the number of school threats has increased since the attack in Parkland, Florida. Within 45 minutes of the incident, law enforcement was also notified and was present on the scene.
A 17-year-old boy threatened to bring a firearm to El Camino High School near Whittier on Friday, prompting an investigation by school authorities and the Los Angeles County Sheriff's Department.
"He was angry with the teacher's issue about his (earphones) in class and he was not allowed to go to her class the next day, so he was supposed to go to the office, " Chavez said.