A security officer overheard a student threaten to open fire at his Southern California high school, allowing officials to thwart the plot just days after a deadly shooting in Florida, authorities said Wednesday.

The 17-year-old student at El Camino High School 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 security officer on Friday heard 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.

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, 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 been increasing in the area since a shooting last week at a Florida high school killed 17 people, 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. 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,” Jacobsen said. “In this day and age, we have to be proactive and make that report and go from there.”

He said the safety of students and staff is the highest priority.

“Given shootings that happened in Florida, and we hear about quite a few of them, we’re all looking to make sure we can prevent these concerns and find out what’s going on,” he said.

Superintendent Hasmik Danielian said in a statement that “we responded quickly and effectively when we first learned about the potentially dangerous threat that was made by the student.”

“We will remain vigilant in our efforts to make sure that we are doing everything possible when it comes to safety and security for our entire school community,” she said.

(AP)