SAN FRANCISCO - Emails containing threats of violence sent to schools in the San Francisco Bay Area in northern California forced at least four of them to shut down Monday, police authorities said.

The Emery Unified School District in Emeryville, about 13 km northeast downtown San Francisco, decided to close schools after receiving the threatening emails Sunday.

"Parents received their first round of robocalls Sunday night and another Monday morning informing them of the decision to close the school," local TV KRON4 reported.

"It was an email that went to a district employee last night saying there were threats of shooting at the school today," said the TV news outlet.

The school authorities have informed local police of the threat, and the U.S. Federal Bureau of Investigation (FBI) is getting involved and investigating who sent the threats to several Bay Area school districts, according to the TV report.

"The FBI is working with our local law enforcement partners to determine the validity of the reported school threats within our area of responsibility. At this time, they appear to be part of a larger influx of threats and they do not appear to be legitimate," the FBI San Francisco Office twitted Monday afternoon.

Emeryville police believe the emails all started through an online gaming community and sort of a contest to see who would generate the most school evacuations in Britain and the United States.

"Not just Bay Area ... they're across the state as well and across the nation," a police officer told the KRON4 TV.

Schools in Emeryville will resume Tuesday following a spring break, but there will be a heavy police presence in case parents or kids have questions or concerns about safety, it said.

School authorities have been on alert about campus violence in recent days following a mass shooting in a high school in Florida on Feb 14, when a 19-year-old gunman shot and killed 17 people and injured another 14.

The massacre has sparked nationwide campaigns against gun-related violence, which culminated in massive protests and demonstrations calling for gun control across the country on March 24.