The Los Angeles County Sheriff's Department said a shooting at a South Whittier high school was thwarted last week after a security guard overheard a student discussing an attack.
Marino Chavez said he heard a 17-year-old male student say he was "going to shoot up the school sometime in the next three weeks".
Los Angeles County Sheriff Jim McDonnell said Wednesday the deputies seized two AR-15s, two handguns - a Colt.45 and a Colt.38 - 90 high-capacity magazines and ammunition. His older brother, Daniel Barcenes, a military veteran who claimed the guns belonged to him, was arrested on charges of possession of an assault weapon, thumbhole stock, import of high capacity magazine, possession of high capacity magazine, criminal storage of a firearm and failure to register a personal handgun.
The alleged threat at El Camino High School occurred on Friday, two days after the shooting at Marjory Stoneman Douglas High School in Parkland, Florida, that left 17 people dead.
Superintendent Hasmik Danielian said in a statement that "we responded quickly and effectively when we first learned about the potentially risky threat that was made by the student".
He said the safety of students and staff is the highest priority.
The school is about 13 miles southeast of Los Angeles. The weapons were found in the home unsecured near the loaded magazines, according to the deputy. The teenager was arrested on suspicion of making criminal threats.More news: Down to the wire for golden Goggia
The Norwalk La Mirada Unified School District released a "Safety Message" on its official website Tuesday, in which the case was not mentioned.
Jacobsen added that when questioned by officials, the student didn't have a weapon on him.
School threats since the Florida shooting include dozens on social media, about 10 incidents where a gun was brought to school and 22 bomb threats, said Amy Klinger, director of programmes for the Ohio-based Educators School Safety Network.
"The sheriff's department can only respond if they are told", Chavez said. However, police said one AR-15 was registered to the brother, but the other was unregistered, which is a felony in California.
"No, I'm not a hero, I'm just doing my job every day", he said.
When the period bell rings, Chavez strolls the campus of El Camino High School in Whittier and ushers lingering students to class. She in turn reported it to authorities and the woman's son was detained, according to McDonnell.
"While these rumors are unsubstantiated, we take them seriously and are working with law enforcement to actively investigate", Steinhauser wrote in a warning published on the school's website.