The FBI warned 18,000 agencies to watch for people wearing bulky jackets on warm days, smelling of chemicals, or even people whose fists are tightly clenched. Also suicide bombers could be disguised as law enforcement officials or pregnant women.

Though there is no hard evidence that terrorists are planning to launch suicide attacks in the United States, the FBI has concerns since extremist rhetoric following reports of abuse at Iraq's Abu Ghraib prison.

Also of concern are upcoming special events including the official opening of the World War II memorial in Washington, the July Fourth holiday and the Democratic and Republican conventions.