A student was later taken into custody, according to the Los Angeles County Sheriff's Department.
Lone Hill Middle School and San Dimas High School were placed on lockdown around 9:30 a.m. as authorities searched the high school, where the caller claimed the armed person was seen walking.
Sheriff's officials said the claim, which came from an anonymous caller, was unfounded and no injuries were reported.
All clear at San Dimas high school. There were anonymous calls from one source about a person with a gun on campus. No weapon was found, there was no actual sighting of a person with a gun. There were no injuries.— LASD San Dimas Stn. (@SDMLASD) August 22, 2019
The Bonita Unified School District tweeted around 10:30 a.m. that authorities believed the report was from a prank call, but the threat was still investigated.
This morning, SDHS & LHMS were placed on lockdown. All students and staff are safe. We are coordinating with the San Dimas Sheriffs. At this time, they are indicating it was a prank call but are taking all necessary precautions for the safety of our students and staff.— Bonita USD (@BonitaUSD) August 22, 2019
AIR7 HD was over San Dimas High where dozens of parents and students were seen outside of the school.
The school district tweeted around 11:15 a.m. that the lockdowns had been lifted at both schools.
"We have confirmed to a satisfactory level that the call was a hoax, and that the call came from a student," the Sheriff's Department said in a statement. " We are prohibited by law from identifying juveniles arrested or detained, so we will be unable to do so in this case."