Thousands of high school students in two US states returned to classes after 48 hours of uncertainty from baseless claims of potential violence on their campuses.
Several threats to schools in California's heavily populated San Gabriel Valley, east of Los Angeles, were received on Tuesday, while schools in at least a dozen Colorado districts went on temporary lockdown or secure mode early Wednesday after receiving a spate of phone threats from unidentified sources, Xinhua news agency reported.
In both states, the threats resulted in administrators locking doors to protect students, the mobilisation and employment of police SWAT (Special Weapons and Tactics) teams, and desperate parents scrambling to determine the safety and well-being of their children.
In Colorado, school officials said the list of impacted districts on Wednesday covered hundreds of kilometres, from Boulder and Englewood on the Front Range to Aspen in the heart of the Rocky Mountains.
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No injuries were reported, and some lockdowns were lifted in a matter of minutes, local media reported.
As was also the case in California, "none of the threats have been deemed credible", according to Colorado's Office of Emergency Management.
"Our State Watch Center staff continue to monitor and communicate with local law enforcement to coordinate information related to today's school incidents," the office said in a statement on Thursday.
According to the Federal Bureau of Investigation, its Denver office and local law enforcement agencies were continuing to investigate the source of the calls, and law enforcement and school districts "believe the threats across the state were coordinated."
"It is important to note that law enforcement will use all available resources to investigate a threat until we determine whether it is real or not," the FBI told local media. "We urge the public to remain vigilant, and report any and all suspicious activity or individuals to law enforcement immediately."
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In the Golden State, Alhambra High School was briefly placed on lockdown Tuesday afternoon as authorities investigated an anonymous threat.
The lockdown was lifted an hour later after police swept the school and quickly debunked "concerns regarding a photo that was airdropped to some students," Fox11 News reported.
"No signs of any threat were discovered during a campus sweep by the police," the school's social media account posted. "The release of students was then coordinated by officers and school administrators."
Fourteen kilometers away at Arcadia High School, another threat resulted in a police inquiry, but that the availability of any additional information was delayed.
Earlier Tuesday, El Camino Real High School in nearby Woodland Hills was placed on lockdown after a report of "a gunman on campus".
The threat was deemed not credible by the Los Angeles Police Department and the lockdown was soon lifted.
"The pattern of this kind of prank-like behavior has increased causing issues of safety in the school," the Gossips World news site said Thursday.
Many parents are thinking of getting their kids homeschooled for their safety issues, but what they learn in school with peers is completely different from what they learn being homeschooled, the news site added.
According to Colorado Public Radio, the activity is called "swatting," with the goal of drawing an armed police response "which may include a special weapons and tactics (SWAT) team."
Swatting is a "hoax call placed to 911 that falsely reports an emergency such as an armed intruder, active shooting or bomb threat", Wednesday's Denver Post said.
There has been a surge in swatting calls targeting schools across the country over the past couple of years, NPR reported.
So far in 2023, there have been 317 false reports of violence in schools around the country, and there has been a "600 per cent increase in the last four years", according to the Educator's School Safety Network that tracks such incidents.
An NPR analysis last year found schools in 28 states were targeted by the same individual.
Most of the attacks, especially those that occur in clusters, "appear to be perpetrated by someone overseas," and the "perpetrator often attacks groups of schools in the same state on the same day", the analysis said.
"They appear to be intended to cause mass panic," according to the National Association of School Resource Officers (NASRO). The motive for these continued attacks, however, is uncertain.