Police received a tip on May 21 of the teen’s alleged plot targeting Berkeley High School, which also included a possible bombing, and obtained a search warrant for his home, Berkeley Police said in a statement.
The search turned up parts to explosives and assault rifles as well as several knives and electronic items that could be used to create weapons, police said.
Authorities also prepared a mental health response through the city’s Mobile Crisis Team to evaluate the teen.
He turned himself in Monday, police said, and was arrested on suspicion of possessing destructive device materials and threatening to commit a crime which will result in death or great bodily injury.
The Alameda County District Attorney’s Office declined to comment because the person arrested is a juvenile.
The discovery of the reported mass shooting plot comes as the US grapples with a series of mass shootings that have left communities across the country grieving.
Less than three weeks ago, 10 people were killed in a racially motivated shooting at a supermarket in Buffalo, New York.
“Speaking up in this instance allowed for the police to intervene in advance of any harm occurring,” Stephens wrote.
He said the school had been in contact with the police department since the onset of the investigation, which is ongoing.
CNN’s Aya Elamroussi contributed to this report.
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