One person was arrested and classes were initially canceled Monday at San Marino High School after an alleged threat was made against a student and possibly other students, police said.
An "active shooter threat" was received around 6 a.m. Monday, according to the San Marino Police Department. It was not immediately clear if the threat was received via phone, email or social media.
The school was placed on lockdown out of an abundance of caution, according to the high school's website.
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The police department later announced that the school was canceling classes and closing the high school until further notice.
"After investigation, it was determined that there is no existing threat," the San Marino Police Department said in a statement.
One person was taken into custody for criminal threats.
Around 10:30 a.m., students were asked to return to campus for fourth period.
Any question concerning the school district should be directed to San Marino Unified School District at 626-299-7000. Anyone with information concerning the threat is asked to contact the San Marino Police Department at 626-300-0720.