More than 30 governors received letters from an anti-government group this week demanding that they resign within three days or face removal from office.
The letters from the group, Guardians of the Free Republics, do not threaten violence, according to officials in Richmond, Va., and Washington. No arrests have been made.
However, the FBI and Department of Homeland Security warned police across the country that the letters could provoke violent behavior.
"The FBI and DHS are not aware of any immediate or credible threat," a Department of Homeland Security official said in a statement. "Although no specific information to indicate violence is intended as part of this plan, the bulletin was shared with federal, state, local and tribal partners to ensure they are equipped with the tools they need to better recognize behaviors and other indicators consistent with homeland security threats to prevent violence or criminal acts."
In at least three states, Nevada, Utah and Nebraska, security was increased after the letters were received.
In Topeka, Gov. Mark Parkinson's office won't say whether it has received a letter like one sent to most of the nation's governors.
Parkinson spokesman Seth Bundy refused to comment Friday, saying the office doesn't discuss matters related to the governor's security.
Guardians of the Free Republics aspires to restore the U.S. republic by peacefully dismantling parts of the government, according to its Web site.
"The Restore America Plan is a bold achievable strategy for behind-the-scenes peaceful reconstruction of the de jure institutions of government without controversy, violence or civil war," the Web site says.
Bruce Hoffman, a professor at Georgetown University who has studied anti-government groups for more than two decades, said the delivery of the demands — to so many governors — is unprecedented.
"Given how emotional politics has become, it has to be taken seriously," he said.
Mark Potok, an editor at the Southern Poverty Law Center who tracks extremists and hate speech, said the group stems from the "sovereign citizen" movement, including those who do not believe in paying taxes or abiding by zoning or other regulations.