CHINO HILLS, Calif. (KABC) -- A lockdown has been lifted after a mystery caller phoned Chino Hills High School on Tuesday and threatened to "blow up" the school.After the school received a call from an unknown male individual, the principal ordered that the campus be placed on lockdown, San Bernardino County sheriff's officials said. The school has since been determined to be safe, according to the Chino Hills Police Department.Deputies arrived on scene and began sweeping and searching the campus. Students were then moved to an area deemed safe.No suspicious devices were found and students have since been dismissed.Road closures surrounding the school have been lifted.