WHITTIER, Calif. -- Police said Wednesday that they discovered two AR-15 rifles, two handguns, and 90 high capacity military-grade magazines at the home of a student who allegedly planned to shoot up his Southern California high school.

According to the Los Angeles County Sheriff's Department, a security guard at El Camino High School in Whittier overhead a "disgruntled student" threaten to open fire on the school on Friday, just two days after 17 people were gunned down at a Florida high school.

LASD Sheriff Jim McDonnell said his department takes seriously threats of violence on school campuses. School-related threats are on the increase in LA county, and are regularly investigated by his department, he said.

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In the El Camino case, McDonnell said a security guard overheard the student say he was going to shoot up the school sometime in the next three weeks. The student had an "extensive" disciplinary record and officials found that a semi-automatic weapon was registered to the student's home address, McDonnell said.

The student was arrested Tuesday on charges of making criminal threats. The teen's brother, 28, has also been arrested on weapons charges. One of the rifles was registered to the brother, but the other was unregistered, McDonnell said.

The brother claimed responsibility for having the weapons, McDonnell said. The brother said he was in the military and had shipped the weapons over from Texas, where he had been stationed, a deputy said. The weapons were found in the home unsecured near the loaded magazines, according to the deputy.

Marino Chavez, the security officer with the Norwalk La Mirada School District who reported the threat, said he felt it was important to immediately contact the sheriff's department to "possibly prevent another tragic event."

"The sheriff's department can only respond if they are told," Chavez said.

Chavez said it was after lunch break and students were returning to class when he heard the threat about a school shooting within the next three weeks. He approached the student, questioned him and brought him to the office.The student confirmed what he said, but apologized and said he was only kidding, Chavez said.

"I said, well you can't say those things on a school campus," Chavez said.

The student said he was angry with a teacher's issue about headphones in class and because wasn't allowed to go to the teacher's class the next day, Chavez said.

McDonnell thanked Chavez for coming forward, calling him an "unsung hero."

"Any time we can get a chance to prevent something like that from happening, I think we all come away very relieved," McDonnell said.