[cnn-photo-caption image= http://i2.cdn.turner.com/cnn/2010/images/04/02/art.gibbons.gi.jpg caption="Nevada Gov. Jim Gibbons received a threat letter on Monday, his office said."]Washington (CNN) - A domestic extremist group has sent letters to more than 30 U.S. governors demanding they resign, the Department of Homeland Security and the FBI said in an intelligence note.
The letters told the governors to vacate their posts within three days, the note said.
DHS and the FBI said there do not appear to be credible or immediate threats of violence attached to the letters.
The group behind the letters has a "Restore America Plan" that calls for the removal of any governor who fails to comply, the intelligence note said.
While DHS has no information that the removal refers to a specific plan for violence, "law enforcement should be aware that this could be interpreted as a justification for violence or other criminal actions," the note said.
Other steps in the group's plan include "establishing bogus courts, calling of 'de jure' grand juries, and issuing so-called 'legal orders' to gain
control of the state," the note said.
States that have received the letters include Wisconsin, Oklahoma, Utah, Arkansas, Maine, Colorado, Rhode Island, and Nevada.
Gov. Jim Gibbons of Nevada received one Monday, and three more Wednesday, one of those by fax, his Chief of Staff Robin Reedy told CNN.
The group behind the letters believes there should be no federal government, Reedy said.
The letters were not handwritten, but did have some handwritten signatures, Reedy told CNN Thursday.
The office had been warned in advance to be on the lookout for the letter, she said.
Gibbons' office stepped up security at the capitol in Carson City after receiving the letters. Everyone had to enter through one entrance, and an X-ray machine and metal detector were brought in. Boulders were placed in front of the capitol so vehicles could not drive close to the building.
The governor had not seen the letters himself, because he had been in Las Vegas for a few days, Reedy said Thursday.
-CNN's Jeanne Meserve and Carol Cratty contributed to this report.