By Mark Hosenball
WASHINGTON (Reuters) - Federal authorities have circulated a bulletin warning that "anarchist extremists" are preparing to use "violence and criminal tactics" - possibly including homemade bombs - to disrupt the Republican and Democratic national conventions.
The FBI and Department of Homeland Security, in the type of bulletin often issued before major gatherings, said they had "high confidence" based on their analysis of similar events that anarchists planned to disrupt the conventions, which begin next week with the Republican gathering in Tampa, Florida.
The Democratic convention is set for the following week in Charlotte, North Carolina, as the presidential campaigns head toward the November 6 election.
A copy of the seven-page document, dated August 21, was posted on Thursday on Cryptome.org, a U.S.-based website similar to WikiLeaks.
A federal official said the document was authentic and is "very similar" to bulletins released ahead of other large events, such as NATO and G8 summit meetings and the annual meeting of the U.N. General Assembly.
The bulletin says that the most serious threat during the conventions is that extremists could use home-made bombs or incendiary devices.
It urges law enforcement officers to look for signs that anarchists are trying to acquire materials for such devices, though it provides no evidence that this has actually happened.
The bulletin, carrying the label "Unclassified//Law enforcement sensitive," says that anarchists and other domestic extremists "probably lack the capability" to overcome massive security measures that will be in place at the conventions.
But, it warns that violent anarchists could target infrastructure near the convention sites, such as local businesses and transportation systems, as well as law enforcement personnel.
The FBI and DHS say they assess with "medium confidence" that anarchist extremists may arrive from states outside of Florida and North Carolina to "engage in criminal activity" during the conventions.
The bulletin cites FBI reporting that, as of mid-March, people associated with extreme anarchism had "planned to travel to Tampa and attempt to close" bridges in the area during the Republican convention. But it said the FBI did not have further details.
The bulletin also noted social networking sites, Internet chat rooms, message boards and mobile devices provide protesters with easy means "to coordinate and facilitate violence or criminal activity threatening public safety."
(Editing by Vicki Allen)