McDonnell said this was the second threat at El Camino High, a continuation high school at 14625 Keese Drive in unincorporated South Whittier, in a week.
A 17-year-old who had earlier tangled with an El Camino High School teacher about wearing headphones in class mumbled something after lunch about "shooting up the campus" within three weeks, according to KTLA.
The school then reported the teen to authorities, who found two assault rifles in his home, including one that was unregistered, according to the AP.
The alleged planned shooting was set to happen two days after 17 people were gunned down at a Florida high school by a former student. School related threats are on the increase in LA county, he said.
"He did say that he was just kidding, that he did not mean it, " Chavez 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 teen had an extensive disciplinary history at the school, McDonnell added.
The Los Angeles Sheriff's office is giving an update on the case Wednesday beginning at 1:45 p.m. ET. He said they found various kinds of ammunition in the boy's room, one handgun in the hallway with two loaded magazines nearby and the three other firearms in a container in the garage which did not have a door.
A misdemeanor conviction for making criminal threats can draw a year in jail.
Local law enforcement officials said they've seen a surge in tips about potential school shootings in 2018.
The sheriff credited "attentive" school resource personnel - in particular the safety officer - as well as "diligent" investigators, with preventing what he said could have been a risky outcome.
One AR-15 was registered to the older brother and the other was not registered, officials said. I'm just doing my job every day.
"The sheriff's department can only respond if they are told", Chavez said.
Court records say the 18-year-old told investigators he was joking and regretted frightening students.
"A Montana high school student was charged Tuesday with intimidation and assault with a weapon over numerous threats to "shoot up the school". Jacobsen said the comment was overheard by Marino Chavez, a school safety officer.
And Inglewood police on Monday became aware of a social media post "indicating that students attending a specific Inglewood school were at risk of being targeted by a shooter", according to a news release.
Sheriff Jim McDonnell credited "attentive" school resource personnel, as well as "diligent" investigators, with preventing what he said could have been a unsafe outcome.