SHERIFF finds AR-15 rifles after threat.

LOS ANGELES — A security guard overheard a student threatening to open fire at his Southern California high school days after a massacre in Florida, and a swift investigation uncovered two assault-style rifles and nearly 100 high-capacity magazines at his home, authorities said Wednesday.

Although the 17-year-old claimed his comment was a joke and investigators didn’t uncover a specific plot, the head of the nation’s largest sheriff’s office said it shows the need to take every potential threat seriously.

“This is an example of something that could have potentially went down that path, but didn’t,” Los Angeles County Sheriff Jim McDonnell said, referring to the Florida high school shooting last week that killed 17. “As we see these incidents occur one after another, we’re all looking to say, ‘how do we stop this?’”

A national organization that tracks school threats says it’s recorded about 50 a day on average since the Florida shooting, compared with about 10 a day on average.

School security officer Marino Chavez heard the teen say Friday at El Camino High School near the city of Whittier that he was “going to shoot up the school sometime in the next three weeks.” The officer reported the comment to authorities, McDonnell said.

Chavez said he asked the student about the threat, and the teen confirmed that he made it but was just kidding and didn’t mean it. The teenager had apparently been angry because a teacher told him that he couldn’t wear headphones in the classroom, the guard said.

“I’m not a hero. I was just doing my job,” Chavez said at a news conference.

Deputies searched the teen’s home and found two AR-15 rifles, 90 high-capacity magazines and two handguns, including one that was left in a hallway laundry basket, the sheriff said.

The teenager, who had an extensive disciplinary history at the school, was arrested on suspicion of making a criminal threat and was in custody, McDonnell said.

His 28-year-old brother, Daniel Eriberto Barcenas, told deputies that he purchased the guns while serving in the Army in Texas. One of the rifles wasn’t registered, which is a felony in California, McDonnell said.

The brother is facing charges of possession of an assault weapon, importing a high-capacity magazine and other violations.

The brothers lived with their parents, and there was no indication that the parents committed a crime, authorities said.

Information for this article was contributed by Amanda Lee Myers, Janie Har, Andrew Welsh-Huggins and Lisa Rathke of The Associated Press.