On Friday, Segerstrom High School in Santa Ana was locked down while police investigated an anonymous call reporting an armed suspect on campus.
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The lockdown was lifted after a police investigation found no threat - and the caller referred to a classroom that does not actually exist at the school. Officials say several other schools in Orange County were also hit by false threats.
Similar threats have been made at schools in Monrovia and Hollywood in recent weeks, as well as in the Bay Area.
New Jersey schools were hit by a wave of calls on Friday as well. Police departments in Florida responded to a rash of calls on Tuesday.
There appears to be a sharp uptick in such incidents across the country this fall as schools return to session.
A Wired magazine investigation identified more than 90 false reports at schools across the United States in just the second half of September alone.
Many of the incidents appear to be connected, happening within minutes of each other and coming from perhaps the same person or group, the magazine said. The calls may have originated from overseas, although investigators are still trying to determine the origin.
At the same time, some law enforcement officials also worry that social media trends encourage teens to make false threats.
Law enforcement officials in South Carolina earlier this month blamed a TikTok challenge for a series of school threats in that state.
At least 18 schools received false shots fired calls last week, according to South Carolina-based WCIV/ABC4.
Swatting - or intentionally calling in a false threat to a school or other location - can cause disruptions to schools and even dangerous situations, experts say. Armed officers don't know what to expect when they arrive at the location, and students and parents are left in a state of fear.
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In at least one incident, a swatting call resulted in a death - and a long prison sentence for the Los Angeles man who phoned in the false report.
In December 2017 a false threat was called in to police in Wichita, Kansas.
Officers who responded to the home mistakenly shot the resident, thinking he was reaching for a gun.
Later investigation determined the call was made as part of an online gamer playing a prank on another, but using an outdated address for his intended target.
Tyler Barriss of Los Angeles was later sentenced to 20 years in prison for making that call. Authorities said he had made multiple other swatting calls in the past.
Before the Wichita incident, Barriss had been accused of making multiple threats to other locations in California and Florida.
He served time in Los Angeles County jail for bomb threats to ABC7 and schools in the Los Angeles area.
Kansas fatal swatting suspect Tyler Barriss opens up in jail interview
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LA swatting suspect opens up in jail interview
SoCal calls
Earlier this week a threat was made toward Monrovia High School, resulting in the campus, as well as nearby Monroe Elementary, going into lockdown before an investigation found no threat.
In September, an anonymous caller falsely reported six people being shot at Hollywood High School.
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Active shooter report at Hollywood High School deemed a 'hoax,' authorities say