APPLETON – A wave of threats directed at schools across the nation Monday forced authorities to lock down buildings or evacuate students.

The threats, which appear to be driven by automated calls, have been directed at elementary, middle and high schools in states across at least four time zones.

Some districts reported that the calls included a bomb threat while others just described the calls as "threatening."

The calls were reported in California, Colorado, Delaware, Florida, Iowa, Maine, Maryland, Massachusetts, Minnesota, Montana, New Hampshire, Oregon, Pennsylvania, Utah, Vermont, Washington and Wisconsin.

Ken Trump, a national school security expert and president of a school security consulting firm, said Monday's nationwide bomb hoaxes have the hallmarks of swatting.

Swatting, according to Trump, are "highly disruptive" hoax threats that are intended to trigger massive police response. Trump said they are often described as robotic, computer-generated voices that call in threats to schools or police departments.