RIVERBANK (CBS13) — Hundreds of school districts across the country, including dozens in California, received online threats, Monday according to police.

The culprits may be a European group trying to shut down schools as part of an online game.

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“We did some brainstorming about what we could do to make sure all of our kids were safe,” said Riverbank Unified School District Superintendent Daryl Camp.

He said he got the threatening email at 10 Sunday night. He called the local police immediately.

“No specific schools, however, there was a reference to bullying and bombs and guns, but no specific information,” said Camp.

After learning a similar false threat had been sent to all school districts in Stanislaus County, Camp said he felt comfortable to hold classes Monday.

“We really thought based on the information that we had and working together with law-enforcement that Riverbank schools, as they usually are, or a safe place for kids and we wanted education to continue,” said Camp.

But that wasn’t the case in other impacted districts. Classes were canceled in Emeryville.

“Information we’ve obtained, this is a hoax,” said one law enforcement official.

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Tuolumne schools remained open, as did Calaveras county schools and five school districts in the San Diego area, all were emailed similar threats.

“The school is sort of damned if you do, damned if you don’t. They have to react law enforcement has to react,” said Jeff Harp, a former FBI agent.

Police say the threats originated overseas in Europe from an online group playing a game to see who could force the most schools to close.

A group claiming responsibility tweeted last month that they successfully caused hundreds of schools in the United Kingdom to close their doors.

While most schools in the US remained open, many were on high alert.

“Anytime you have a prank that utilizes law-enforcement resources and take some away from where they’re supposed to be it can be a very dangerous thing,” said Harp.

The FBI is working with local agencies on this case. The agency has not verified who sent the messages.

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An FBI spokesperson says they’ll release more information Tuesday.