More than 30 governors, including Republican Robert F. McDonnell of Virginia, received letters from an anti-government group this week demanding that they resign within three days or face removal from office.
The letters, which appeared to be from the group Guardians of the Free Republics, do not threaten violence, according to officials in Richmond and Washington. No arrests have been made.
Maryland Gov. Martin O'Malley did not receive a letter, spokesman Shaun Adamec said Friday.
The threats come after an outbreak of harassment and vandalism began against members of Congress a couple of weeks ago. A Philadelphia man was charged Monday with threatening to kill Republican Rep. Eric Cantor of Virginia and his family.
The FBI and Department of Homeland Security warned police across the country that the letters could provoke violent behavior but said in a statement they were "not aware of any immediate or credible threat."
'Peaceful reconstruction'In at least two states, Utah and Nebraska, security was increased after the letters were received. In Nevada, screening machines were added to the main entrance of the state Capitol.
Guardians of the Free Republics aspires to restore the United States 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 threat to so many governors is unprecedented.
"Given how emotional politics has become, it has to be taken seriously," he said. "On the one hand, it's absurd. On the other hand, given the time it took to contact 30 governors, it's not easily dismissed."
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, abiding by zoning or other regulations or that they have committed a crime if there is no complaining witness.
All might get lettersIn Virginia, the letter was received at the governor's office in Richmond on Wednesday. It was turned over to the Richmond FBI office.
The governor's office released a statement Friday morning saying the governor and his family were safe.
Governors who received the letters include Republicans Tim Pawlenty of Minnesota, Bobby Jindal of Louisiana, and Michael Rounds of South Dakota and Democrats Chet Culver of Iowa, Jennifer M. Granholm of Michigan and Brad Henry of Oklahoma. The FBI expects that all 50 governors will eventually receive such letters.
The FBI and Department of Homeland Security released a joint statement: "DHS and FBI are aware of the letters sent to governors, and while there does not appear to be credible or immediate threats of violence attached to the letters, we are working with state and local authorities and continue to assess the matter."
Washington Post staff writer Spencer Hsu and researcher Eddy Palanzo contributed to this article, which includes information from the Associated Press.