Officers were called to Reedley High School after a phone call threatening that a bomb would be placed on campus.
Video from an ABC30 Insider showed a heavy police presence at the school, including a sheriff's K-9 unit.
The campus was locked down for close to an hour while officers searched the area. No credible threat was found.
Reedley was not the only campus affected. Fresno-area schools, including Fresno High School, Bullard High School, and Clovis High School, also received bomb threats around the same time.
Each campus was placed on lockdown as law enforcement conducted thorough sweeps of classrooms and buildings.
Clovis police said agencies coordinated as the threats came in.
"We shared information with the other agencies that also received these threats today as well," said Sgt. Sean O'Brien of the Clovis Police Department. "I don't have the details of those other calls, but we put out the information we received to see if there are similarities, possibly the same suspects and same threats."
At Clovis High School, police deployed a drone to give officers a birds-eye view of the campus during the search. Authorities said every threat is treated as real until proven otherwise.
"There's nothing that we would take as, oh, this might be fake, we don't want to get into that mindset," O'Brien said. "We want to treat it as if its real until we can prove otherwise."
Investigators said so-called swatting calls have become increasingly sophisticated. O'Brien said callers often use fake phone numbers or caller ID spoofing to conceal their identities.
When calls originate from outside state lines, federal agencies may become involved.
"They can be charged either as misdemeanors or felonies and with that, people can be looking at years in prison and fines and cost recovery for emergency services that are dispatched to those," O'Brien said.
All campuses were eventually given the all-clear, and no injuries were reported.
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