The Federal Bureau of Investigation has issued a warning that fake bomb threats are being emailed to US polling locations in multiple states that “appear to originate from Russian email domains.”

“None of the threats have been determined to be credible thus far,” the FBI says. The agency says it is working closely with state and local law enforcement to respond to any election threats and urges the public to “remain vigilant” and report suspicious activity to state and local authorities.

Fake bomb threats were sent to polling locations in Georgia, Michigan, and Wisconsin, prompting “at least two” sites in Fulton County, Georgia, to be evacuated on Tuesday, according to Reuters. Officials said both locations were “re-opened after about 30 minutes,” the outlet reports. The county has asked for “a court order to extend the location’s voting hours” to after 7PM ET, the state’s deadline for voting.

The threats appear to be part of a larger campaign to sow doubt and chaos in the US election. On Friday, a joint statement issued by the FBI, the Office of the Director of National Intelligence, and the Cybersecurity and Infrastructure Security Agency warned of Russian-created fake videos designed to “raise unfounded questions about the integrity of the U.S. election.” In another statement yesterday, the agencies said Russian “influence actors” were amplifying false claims of US officials planning to “orchestrate election fraud using a wide range of tactics.”

These reports come after years of concern about Russian online interference in US politics. That has included influence campaigns carried out by the Kremlin-linked Internet Research Agency as well as Russian and Iranian acquisitions of US voter registration information that may have been used to threaten people through email if they didn’t vote for Trump in 2020.

Update, November 5th: Updated with details about where bomb threats were sent.