An alert security officer at a Southern California high school helped thwart a potential attack Friday, two days after the deadly Valentine’s Day shooting in Florida that left 17 people dead, authorities said.

The Los Angeles County Sheriff’s Department said they raced to prevent a copycat attack last Friday, acting on a tip from a security officer at El Camino High School in Whittier, southeast of Los Angeles.

The officer, Marino Chavez, overheard a 17-year-old student say he was going to launch an attack within three weeks, he said at a news conference. Chavez questioned the student, who claimed it was a joke, he said.

“He didn’t appear scared, but he was like, ‘Well, I didn’t mean it,’ ” Chavez said. “I said, ‘I know you students say a lot of things. But you can’t be saying these words.’ ”

Investigators detained the student and issued a search warrant of his home, Los Angeles County Sheriff Jim McDonnell said. Authorities seized two AR-15 rifles and two handguns, along with about 90 rifle magazines each capable of holding 30 rounds, though they could not say how many were loaded at the time. An AR-15 was used by the gunman in Parkland, Fla., in one of the deadliest school shootings in history.

McDonnell said he believed the student had an “extensive” discipline history and was “moving in the direction” of an attack.

Chavez deflected credit for stopping a potential school shooting, telling reporters: “I’m not a hero; just doing my job.”

The teen was arrested for making a criminal threat. The student, who is being held without bail, remains unidentified due to being a minor, McDonnell said.

One recovered rifle was registered to the student’s brother, Daniel Barcenas, 28. The other was unregistered — a felony in California, McDonnell noted. Barcenas was charged with five criminal counts Tuesday, including possession of an assault weapon and bringing high-capacity magazines from Texas, McDonnell said. He claimed responsibility for the firearms.

Tips involving school violence received by county authorities went up “significantly” in January, compared with those received the same month last year, McDonnell added. The county has received 19 leads concerning threats involving schools since the Florida attacks alone. There were 52 received in all of 2017, pointing to an uptick in students mimicking threats.

Copycat threats have mushroomed on Snapchat and other social networks, fueling uneasiness on campuses across the country since the shooting in Florida.

Though most appeared to be hoaxes — “jokes,” as several suspects called them — administrators, police and school resource officers have been on high alert, looking out — warily — for students motivated to threaten or engage in similar behavior.

In Arkansas, a student allegedly threatened to “shoot up the high school like they did in Florida,” and was arrested. In southeastern Massachusetts, a social media post warned local high school students of a “Florida pt 2.”

In South Carolina, a ninth-grader was arrested after he allegedly posted a photo of himself on Snapchat wearing a mask and holding what appeared to be an assault rifle. That picture was captioned, “Round 2 of Florida tomorrow,” and alarmed authorities across the state line. The suspect claimed it was a joke.

When confronted by South Carolina deputies, the student in the photo said he posted it in jest.

And Wednesday morning, Long Beach Unified School District Superintendent Chris Steinhauser notified parents of an unconfirmed threat of a potential attack on Long Beach Polytechnic High School.

McDonnell said social media can be  a warning sign — as when someone posts disturbing information signaling a potential attack — or it can be harnessed.

“They live their lives around social media” and can be numb to the seriousness of threats, he said, referring to students.

In a letter posted online Wednesday, Superintendent Hasmik Danielian of the Norwalk-La Mirada school district said officials were cooperating with the police and have maintained regular drills involving active shooters.

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