Washington (CNN) -- There is "no specific or credible information" terror groups are planning to attack the United States during the summer holiday season but law enforcement agencies should remain vigilant, the FBI and Department of Homeland Security say in an intelligence bulletin.
The death of al Qaeda leader Osama bin Laden could spur lone offenders to try an attack on a holiday like July Fourth to "increase the symbolic impact," they say.
In the intelligence bulletin, dated Monday and obtained by CNN, federal officials note that as recently as February 2010 al Qaeda was interested in striking U.S. targets on symbolic dates such as the July Fourth holiday. That information was discovered in the cache of materials seized by U.S. Navy SEALs during their May raid of bin Laden's house in Pakistan.
There is no information the al Qaeda goal led to the development of actual plots against the United States, and it is unclear how many al Qaeda members or other terrorists shared that aspiration, the bulletin says.
Previous law enforcement notices have warned that terrorists could strike during holidays or target events where large numbers of people would gather in one place.
This week's bulletin makes that point and lists "sporting events, parades, religious and cultural activities, retail centers and shopping malls, airports and public transportation systems as especially attractive targets during the holiday season."
The payoff for terrorists would be an "opportunity to inflict mass casualties, with the added objectives of causing economic and psychological damage on the United States," it says.
The FBI and Homeland Security Department provide a list of behaviors that might indicate a person or group is planning an attack. These include trying to get information about security measures at a location, taking videos, watching to see how security personnel react during a drill or false alarm, and showing interest in buying materials that could be used to fashion explosive devices.
Officials also listed protective measures that could be taken, including conducting background checks on employees, developing thorough security and emergency response plans, and installing blast-resistant trash containers.
The bulletin lists several examples of terror attempts, including the May 2010 attempt to detonate a car bomb in the crowded Times Square area.