SALT LAKE CITY - Security is tight this week after Utah Gov. Gary Herbert received a letter that threatened to physically remove him from power. The Department of Homeland Security and the FBI issued a note about the group calling itself "Sovereign Citizens," allegedly plotting to remove state governors from around the nation. Gov. Herbert and Nevada Gov. Jim Gibbons received letters Tuesday dem More..anding they resign or be forcibly removed.  According to the Dept. of Homeland Security, Sovereign Citizens claim that they are not subject to most taxes, are not citizens of the United States, but are instead non-resident aliens. The letters do not specifically threaten violence, but a spokeswoman for Gov.Herbert tells Fox 13 Thursday that they did forward it on to the FBI, which confirms they are investigating the letter. Some other states have restricted access by shutting down some entrances and installing additional x-ray machines.