At least a half-dozen universities across the country, including the University of New Hampshire, welcomed students to fall classes Monday 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 was found.

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

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