In recent months, hoaxers playing online games have allegedly used proxy servers and other high-tech identity-disguising tools to anonymously threaten schools online or in phone messages with electronic voices to trigger a huge police response, including SWAT teams.

The latest threats led to the evacuation of schools in Colorado, Utah, Delaware, Minnesota, New Hampshire and Wisconsin. Media outlets in the United Kingdom also reported evacuations.

Students at Murray High School outside Salt Lake City were sent home around noon. Fox 13 News in Salt Lake City reported law enforcement searched the school and bomb sniffing dogs were brought in. No explosives were found.

Murray School District officials posted updates on the district’s Facebook page, letting parents know law enforcement determined no credible threat was found and the school would be open for evening activities.

The threats were made against elementary, middle and high schools, with some schools choosing to continue classes and others to put buildings on lockdown rather than evacuate. Some schools resumed classes after sweeps by authorities failed to turn up explosives or other threats.

Some officials described Monday’s threats as automated or robotic and at least two — at Lakewood High School outside Denver and at Ben Franklin Elementary School in Rochester, Minnesota — came in just before noon local time. Also in Minnesota, Forest Lake Elementary in the city of Forest Lake was evacuated after getting a bomb-threat call around 12:15 p.m.