Scott Iskowitz / AP

Republican National Committee Chairman Reince Priebus, left, and convention CEO William Harris unveil the stage and podium for the 2012 Republican National Convention on Monday in Tampa, Fla.

A federal law enforcement bulletin "assesses with high confidence" that extremists from anti-government anarchist groups will try to use violence with the aim of disrupting both political conventions.

That assessment, it says, is based on what has happened at similar high-profile events.

The analysis, sent this week to local police organizations nationwide by the FBI and Department of Homeland Security, says these groups "probably lack the capability to overcome the heightened security measures" around actual convention facilities but could target businesses or means of transportation nearby. Most of what these groups do, the bulletin says, involves vandalism, trespassing and attempts to block roads. But it says they're becoming more interested in trying to use explosives.

The Republican national convention is scheduled to run Monday through next Thursday in Tampa, Fla. The assessment says the FBI picked up information in mid-March indicating that some anarchists from New York were planning to go to Tampa in hopes of closing "all the Tampa Bay-area bridges" during the convention, although it doesn't say how they planned to do it.

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Though the bulletin makes no mention of Tropical Storm Isaac, it says that "inclement weather and effective crowd control" could deter the actions of these groups.

Isaac was in the Caribbean Sea on Thursday and expected to pass south of Puerto Rico. Some computer models showed Isaac moving parallel to Florida's western coastline, passing Tampa on Monday. 

Pete Williams is NBC News' chief justice correspondent.

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