Conventions warned about anarchists

• Intelligence bulletin prepared ahead of national political conventions
• It warns of possible activity by anarchist groups
• Bulletin cites concerns over bridges, infrastructure

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.