School shooting threats made on TikTok and other social media prompted school officials and authorities across the U.S. on Thursday to step up security measures.

Driving the news: Law enforcement agencies believe the threats that warn of possible incidents this Friday are not credible, but schools in D.C., New York, New Jersey, California, Michigan, Maryland, Virginia, Minnesota and elsewhere have taken precautions including closing for the day or increasing police or security presence.

Why it matters: The threat is rattling students on what for many is the last school day of the year, with exams and Christmas fun on the docket. Instead, schools are cutting back and locking down, with last month's attack in Michigan renewing fears of shootings.

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.

— Governor Phil Murphy (@GovMurphy) December 16, 2021

Context: Officials in Tooele County, Utah, said in a statement that they believe the "challenge for students post a threat regarding gun violence in schools on social media" originated on TikTok, but it's also been seen on Instagram and Facebook.

• "Multiple outlets have stated the original threat started off as a way for students to skip school or be excused from school and has morphed into something much more disturbing," the statement added.

Of note: Police in Seattle have opened investigations into threats at two schools, "which led to the closure of one high school campus," per a statement issued Wednesday.

• Oxford Community Schools in Michigan, where four students died during a school shooting last month, has already closed its buildings for the rest of the week due to the threats.

Meanwhile, Michigan's Charlton Police Department issued a statement Thursday saying it's "aware of a Snap Chat message about a threat to Charlton Middle School."

• "Our agency is actively working with other agencies and the school," the statement continued.
• "We believe this threat may be related to Tik Tok challenge and not a credible threat. We have patrols concentrated in certain areas as a precaution."

The University of Wisconsin at Oshkosh said: "We do not believe the threat is credible."

• Oshkosh K-12 schools said in a letter to students, families and staff: "[T]here is no information that these warnings are directed toward any Oshkosh schools ... However, in an abundance of caution, there will be increased police presence."

What they're saying: TikTok in a statement Thursday said, "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 FBI said in a statement to news outlets that it "takes all potential threats seriously and we regularly work with our law enforcement partners to determine the credibility of any threats."
• Authorities in several cities have warned that students could be disciplined or prosecuted for making threats.

Sandy Hook Promise, a nonprofit organization founded by families who lost loved ones in the Sandy Hook school shooting of 2012, tweeted:

• "A national TikTok challenge promotes school shootings on December 17. Gun violence is not a subject for jokes or pranks."

By the numbers: Schools have been subjected to nine active and 235 non-active shooter incidents this year, per the Center for Homeland Defense and Security,

Editor's note: This story has been updated with new details throughout.