California High School in San Ramon was placed on lockdown Thursday after campus staff received a phone call threatening violence, police said.
Officers responded to the campus, set up a perimeter, and searched the school with the help of explosive detection K9s.
Police at 12:45 p.m. reported the lockdown lifted after the campus was cleared.
The school will remain in a "secure" campus mode, which means students can move throughout the interior of the campus, but outside individuals will be kept off campus, according to police.
Officers will also remain on campus for the rest of the school day out of an abundance of caution.
An investigation into the threat is ongoing.
Officials are also providing updates on San Ramon Police Department's Live Alerts page.
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