Alleged posts on TikTok warning of shooting and bomb threats have prompted several schools across the United States to cancel classes or increase police presence on campus. Districts in the states of California, Texas, Minnesota, and Missouri said schools would be shut on Friday, local media reported. Security has been stepped up for students and teachers in Arizona, Connecticut, Illinois, Montana, New York and Pennsylvania. The purported threats come in the aftermath of a sophomore killing four students and wounding another six and a teacher at a high school in Michigan on November 30. The 15-year-old suspect was later charged as an adult for crimes including murder and terrorism.

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A report in The Verge said there is “little evidence that the threats are credible — or even exist.” Police had spoken of a “trend referencing the possibility of shootings or bombings on December 17th”, the report said, “but it’s not clear how many have seen a specific threat or a threat against their schools in particular.”

The Verge report quoted just one confirmed instance of a police department actually viewing one of the purported threatening messages. However, police in Gilroy, California, had subsequently determined that the initials mentioned in the social media post was actually about a school different from the one that was initially thought to be under threat.

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The Verge said that the Los Angeles School Police Department had identified who had posted it, and determined that the person was not a “credible threat”. Gilroy police didn’t say whether the post was made on TikTok, the report said.

Michigan State Police said in a statement on Thursday that it was unaware of any credible threats.

Baltimore County Public Schools (Maryland) said on Twitter that “law enforcement agencies have investigated… an anonymous threat posted on TikTok targeting all schools in the United States”, and “determined that it originated in Arizona and is not credible.”

But there is an “abundance of caution.”

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Governor Phil Murphy of New Jersey said on Twitter that “no known specific threats” existed, but the state would nonetheless stay “prepared”.

“While there are no known specific threats against New Jersey schools, the safety of our children is our highest priority and we will work closely with law enforcement to monitor the situation and remain prepared,” Murphy tweeted.

TikTok said it took even “rumoured threats” seriously.

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“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 posted on Twitter.

An email sent to parents by administrators of the Oak Park and River Forest school in Illinois: “We are writing to inform you and not alarm you… 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.”

Administrators said that local police would increase their presence around schools “out of an abundance of caution.”

(With AP inputs)