Washington (CNN) -- Law enforcement officials are concerned about possible violence by anarchist extremists at the upcoming Republican and Democratic national conventions, according to an intelligence bulletin prepared by the FBI and Department of Homeland Security.
The bulletin, which was obtained by CNN, says that anarchists could try to use improvised explosive devices.
It also says that, as of March, the FBI had intelligence indicating individuals from New York "planned to travel to Tampa and attempt to close" all of the Tampa Bay-area bridges during the Republican National Convention next week.
According to the document, the FBI's information as of March showed that anarchist extremists proposed "engaging in potentially destructive criminal activities against critical infrastructure outside the security perimeter throughout the Tampa Bay region because they expected access to the main RNC venue to be tightly controlled."
The FBI routinely issues intelligence bulletins in advance of many major events to warn local law enforcement of possible security concerns.
The document notes anarchists have a history of trying to disrupt major events.
"During past national and international political and economic events, anarchist extremists have blocked streets, intersections, and bridges to disrupt or impede local business operations and public transportation access and, in some instances have initiated violent confrontations with police," the document states.
The agencies said that before the 2008 Republican National Convention in Minneapolis, anarchists discussed blocking bridges and sky walks, taking over a radio station, targeting corporations and identifying hotels where delegates were staying.
The anarchists also talked about identifying the location of police stations and emergency routes, according to law enforcement officials. The document, however, doesn't provide details of what anarchists allegedly wanted to do with that information.
"FBI and DHS assess with high confidence anarchist extremists will target similar infrastructure in Tampa and Charlotte, with potentially significant impacts on public safety and transportation," according to the law enforcement alert. The Democratic National Convention is being held in Charlotte, North Carolina, in early September.
The bulletin says that law enforcement agencies believe most protesters at the conventions will obey laws and not commit violent acts, but that anarchists are the most likely exceptions.
The notice warns of certain things state and local law enforcement should be on the watch for. Those include anarchists acquiring materials that could be used to make improvised explosive devices, obtaining firearms training and preparing for violence with groups they oppose -- such as white supremacists.
Anarchists often research potential targets, according to the bulletin.
Law enforcement officials cite the arrest in late April of five Ohio men for allegedly plotting to blow up a bridge over the Cuyahoga River near Cleveland. Authorities said at least three of the men are self-proclaimed anarchists who had considered "a series of evolving plots over several months."
The bulletin says the suspected anarchists conducted surveillance and did research on the Internet. The five pleaded not guilty to the charges.
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