No explosive devices were found at any of the properties, police said. It's unclear if the series of bomb threats are connected, and the incidents are under investigation.
The first threat was reported around 1:25 p.m. at Troy High School. In the span of about an hour, there were three more reports at Fullerton Union High School, Sunny Hills High School and Fern Drive Elementary School.
"The Fullerton Police Department deployed multiple resources to all four schools, including assistance from outside agencies and the use of explosives-detection canines to search the campuses," police said. "Officers worked closely with school administrators to ensure the safety of students, staff, and surrounding communities."
Just after 5 p.m., officers responded to St. Jude Medical Center after a caller claimed there was a bomb inside the emergency room. Police assisted in a partial evacuation as a precaution, and the patients were later allowed back inside the emergency room after no threat was found.
Police said detectives are working to identify who made the threats.