Palo Alto Unified became the latest school district to receive an anonymous threat of violence in an email sent to interim Superintendent Karen Hendricks late Thursday evening.
Hendricks reported the threat, which was not specific to the district or any of its schools, in a weekly message Friday afternoon. She wrote that the "anonymous email threat was similar" to ones school districts throughout California and the country have received in recent days.
Dozens of districts in Santa Clara, Alameda, Contra Costa, Sonoma and Napa counties have received threats, according to news reports. Some schools were placed on lockdown or canceled classes as a result.
Hendricks said she immediately reported the email she received to the Palo Alto Police Department.
"It is believed that the email was again distributed throughout the state and nation; law enforcement continues to work to determine the origin of the emails," she wrote.
Police spokesperson Zach Perron said that Palo Alto Police investigators "determined that it was related to the nationwide series of hoaxes" but do not know where the threat originated.
Palo Alto Police Officer Mike Foley said Friday night that school resource officers were "visible" on campuses after the threat and police sent additional patrol officers to cover the schools.
The FBI's San Francisco office is working with local law enforcement to "determine the validity of the reported school threats within our area of responsibility," the agency tweeted on Monday.
"At this time, they appear to be part of a larger influx of threats and they do not appear to be legitimate."
The Bay Area News Group reported Wednesday that a Swiss cyber group claimed responsibility for "launching a massive school-violence hoax emailed to superintendents across the Bay Area, California and the rest of the country Sunday and Monday, according to an online gaming executive."
A Dallas-based online gaming company, Zonix, reportedly said its domain name was hijacked by Apophis Squad, which also took responsibility for sending hundreds of threats to United Kingdom schools last month.
The local threat comes just two weeks after Palo Alto High School was placed on lockdown for more than an hour as police investigated a shooting threat directed at the school. It was later determined to be a hoax.
Hendricks said the district will "continue to actively look into any communication that voices threat to school safety, and that our staff will continue to be vigilant."