At least a half-dozen universities across the nation on Monday welcomed students to first-day-of-fall classes with run-and-hide warnings about possible gunmen on campus.

In almost all the cases, police and administrators said the reports were hoaxes or swatting calls, which is when someone uses temporary cellphone numbers and voice-cloaking apps to create havoc. A few campuses said simply that no evidence of a gunman or violence was found.

The reports were sent to students at the University of Arkansas, Colorado State University Boulder, Iowa State University, Kansas State University, University of New Hampshire and Northern Arizona University, according to student alerts and school statements.

ditionally, according to campus officials, the University of South Carolina received two reports of an active shooter at Thomas Cooper Library in Columbia on Sunday night; the University of Tennessee-Chattanooga received a hoax active shooter call on Aug. 21; and Villanova University received two false active shooter reports during freshman orientation last week.

The University of Arkansas canceled classes Monday after determining reports of “active threats” could not be confirmed. “Students are free to leave campus at this time if desired,” university police said.

Kansas State University described a false report Monday at its Manhattan campus as “similar to other swatting reports happening at universities across the country.”

University of South Carolina police said in a statement Monday that both false reports from Sunday night appeared to be carried out by the same male and triggered a massive response that included “mutual aid,” or officers from surrounding communities.

“Both calls were initiated by an unknown male and included background noise that mimicked gunfire,” police said.

Northern Arizona University said in a statement that a caller reported a gunman Monday at Cline Library on its Flagstaff Mountain Campus, triggering a response that included Flagstaff police, Coconino County Sheriff’s Office deputies and state and federal agents.

“The report was determined to be a hoax, and at no time was there an active threat to the NAU community,” the university said in a statement. “An investigation is underway into the false report, with assistance from the Federal Bureau of Investigation.”

An FBI spokesperson said the bureau was aware of many of the reported incidents and was helping with investigations.

“The FBI continues to work with state and local partners to investigate the swatting hoaxes but cannot say at this stage whether or not the incidents are connected,” the bureau said.

Some of the earliest swatting incidents from the 2010s involved false shooting reports at the homes of celebrities, and in recent years, they’ve expanded to include politicians and institutions. The FBI created a national database to track such fake calls.

Last year, authorities alleged a California teenager carried out hundreds of swatting calls that targeted historically Black colleges, high schools, the homes of FBI agents, and a Florida mosque in a spree he ultimately admitted carrying out.