WASHINGTON -- A series of letters that claim to be from an anti-government group warning more than half the nation's governors that they must leave office immediately has caught the attention of federal and state authorities, who are concerned that the letters may provoke others to use violence.

As of Wednesday, more than 30 governors had received letters saying if they don't leave office within three days, they will be removed, according to an internal intelligence note by the FBI and the Department of Homeland Security that was obtained by the Associated Press.

The letters are coming from a purported group called the Guardians of the free Republics, which has a Web site that lays out a plan to "restore America" by peacefully dismantling many government institutions.

The FBI is associating the letter with so-called "sovereign citizens," most of whom believe they are free from all duties of a U.S. citizen, such as paying taxes or needing a government license to drive. A small number of these people are armed and could resort to violence, according to the intelligence report warning the governors about these letters.

The Web site claiming to represent the group calls for an end to foreclosures, taxes, auto registrations and birth certificates, among other things.

It advocates what it calls behind-the-scenes peaceful reconstruction of the institutions of government.

The letters follows an FBI raid on suspected members of the "Hutarees," a radical militia group in the Midwest, who are accused of planning to kill police officers. Nine members of the group face various charges, including conspiracy.

In the past year, federal agents have seen an increase in "chatter" from an array of domestic extremist groups, which can include radical self-styled militias, white separatists or extreme civil libertarians and "sovereign" citizens.

In the case of the letters to the governors, there is no direct threat of violence, but the FBI is concerned that the demand could encourage others to act out violently. 

In explaining the letters sent to the governors, the FBI intelligence note says officials have no specific knowledge of plans to use violence, but they caution police to be aware in case other individuals interpret the letters "as a justification for violence or other criminal actions."

An FBI intelligence note is an informal warning that advises law enforcement what to expect. It's not as serious as an intelligence bulletin, which usually contains specific information about a threat.

In most cases, the letters to the governors are being sent directly to state police authorities.

"We're aware of the letters being sent to governors," one security official told Fox News. "There is no credible or immediate threat, but we are working closely with local law enforcement officials."

Gov. Bobby Jindal's office confirmed the governor had received a letter from the Guardians of the free Republics and directed all further questions to the Louisiana State Police. 

Lt. Doug Cain of the Louisiana State Police, which has the job of protecting the governor, said the letter to Jindal was "not a credible threat to the governor or to the people of Louisiana." He told Fox News that the governor had received the letter earlier this week, and they contacted the state police. Cain said no crime was committed in the letter, but they were still investigating it and to his knowledge several states had received it.

Michigan Gov Jennifer Granholm received the letter Monday, nearly five days ago, which renders the letter's three-day deadline as apparently meaningless.

Granholm spokeswoman Liz Boyd said federal authorities had alerted the governor that such a letter might be coming. Boyd, who described the letter as "non-threatening," said it was opened by a staffer and immediately turned over to the Michigan State Police. 

Other governors who have received the letter include Ed Rendell of Pennsylvania, Jim Doyle of Wisconsin, Mark Sanford of South Carolina and Jim Gibbons of Nevada.

Authorities in Nevada installed screening machines for visitors at the state Capitol in Carson City as a precaution.

"We're not really overly concerned, but at the same time we don't want to sit back and do nothing and regret it," Deputy Chief of Staff Lynn Hettrick said. 

Fox News' Steve Centanni, Mike Levine and The Associated Press contributed to this report.