A spokesman for the California school district said the student had been unhappy with a rule banning headphones in the classroom when he was heard mumbling about returning with a gun.
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. "They also learned that a Smith & Wesson semi-automatic weapon was registered to his home address".
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.
A few hours after Chavez raised the alarm, Los Angeles County sheriff's investigators found a cache of firearms, including two semiautomatic rifles, two handguns and 90 high-capacity magazines, at his home, reported the Los Angeles Times.
The teen was arrested on suspicion of making criminal threats.
Robert Jacobsen, an attorney for the Norwalk-La Mirada Unified School District, told reporters the student got into an argument with a teacher over a set of headphones.
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El Camino High School in Whittier, Calif., where authorities say a "disgruntled student" planned a mass shooting.
"They felt there was enough there that they should call law enforcement so they can investigate further", Jacobsen said.
Jacobsen said he thought the student had been arrested, but a sheriff's detective would not confirm whether that had happened as of Tuesday.
The security officer notified school officials, who contacted the sheriff's department.
"He did say that he was just kidding, that he did not mean it, " Chavez said.
The teen allegedly made the threat while at the continuation high school campus at 14625 Keese Drive, which is in the unincorporated community of South Whittier. 'I said, "Well, you can't say those things on a school campus"'.