A 17-year-old student who threatened to open fire at his California school has been arrested after weapons were found at the home he shared with his army veteran brother.

The El Camino High School student near the city of Whittier was arrested on suspicion of making criminal threats, and his adult brother was arrested on five weapons charges after two assault rifles, 90 high-capacity magazines and other handguns were found in their home.

A school security officer on Friday overheard the teen “say that he was going to shoot up the school sometime in the next three weeks”, Los Angeles County Sheriff Jim McDonnell said at a news conference on Wednesday local time.

The school reported the teen, and deputies found an assault rifle at the home registered to his 28-year-old Army veteran brother and another that was not registered, which is a felony in California, Sheriff McDonnell said.

The sheriff said the brother is facing charges of possession of an assault weapon and other violations, and that the teen had an extensive disciplinary history at school.

School threats have increased in the area since a shooting last week at a Florida high school killed 17 people, Mr McDonnell said, adding that “this should be a wake-up call for all of us”.

School district security officer Marino Chavez told reporters that when he heard the threat, he asked the student about it, and the teen confirmed that he made it but was just kidding and didn’t mean it.

Mr Chavez told the student he could not say such things at school.

Robert Jacobsen, general counsel for the Norwalk-La Mirada Unified School District, said the teen didn’t like a teacher’s rule banning headphones in class but declined to provide further details about him, citing privacy issues.

“They felt there was enough there that they should call law enforcement so they can investigate further,” Mr Jacobsen said.

“In this day and age, we have to be proactive and make that report and go from there.”

– AP