Threats of school shootings and other acts of violence have caused schools across the country to either cancel class or increase security. According to reports, the threats, which started circulating on TikTok, singled out Friday (Dec. 17) as “National School Shooting Day.” The threats did not name any specific schools or districts, nor have police deemed them to be credible. However, educators are exercising caution.

“We are writing to inform you and not alarm you,” administrators at schools in Oak Park and River Forest, Illinois wrote in an email to parents. “We have been made aware of a nationwide viral TikTok trend about ‘school shooting and bomb threats for every school in the USA even elementary’ on Friday, December 17.”

According to authorities, threats of violence and copycat crimes have increased since the deadly shooting at a Michigan high school earlier this month. Schools in Arizona, Connecticut, Illinois, Montana, New York and Pennsylvania said Thursday (Dec. 16) that they would increase police presence in light of the threats.

“We have partnered with local law enforcement to vigilantly investigate whether there are any local connections to these social media posts,” the Iowa City Community School District wrote in an email, per USA Today. “At this time, law enforcement has not acquired any evidence that would deem these threats as credible in our area. Out of an abundance of caution, our schools will be in continual contact with law enforcement throughout the day on Friday.”

Other schools, like California’s Gilroy High School, opted to cancel class on Friday instead.

On Twitter, TikTok wrote that it was working with law enforcement to investigate the threats.

“We handle even rumored threats with utmost seriousness, which is why we’re working with law enforcement to look into warnings about potential violence at schools even though we have not found evidence of such threats originating or spreading via TikTok,” the company wrote.

In a statement to CBS News, an FBI spokesperson also said, “[The FBI] takes all potential threats seriously. We regularly work with our law enforcement partners to determine the credibility of any threats. As always, we would like to remind members of the public that if they observe anything suspicious to report it to law enforcement immediately.”