WASHINGTON - The Indianapolis 500 is an attractive target for terrorists, federal authorities said, but there has been no credible or specific threat aimed at the Memorial Day weekend race.

In an internal FBI/Homeland Security Department assessment released Monday to local police, officials said such sporting events, which attract hundreds of thousands of spectators, are attractive potential targets. The assessment said these popular events are inviting to terrorists because of the potential to inflict large numbers of casualties while the whole world is watching.

The assessment is supported by a congressional report, expected to be released this week, about the threat to mass gatherings. The report, written by the democratic staff on the House Homeland Security committee, finds that major events — such as NASCAR races, the Super Bowl and presidential nominating conventions — are all attractive targets to terrorists. The report also raised the potential for infectious disease outbreaks and other bioterror threats at such events.

Rep. Bennie Thompson, D-Miss., said first responders and private organizations are working hard to secure these events and prepare for mass causalities. But Thompson, chairman of the House committee that wrote the report, said in a statement Monday, “Without increased federal support and guidance, mass gatherings will remain vulnerable.”

Indianapolis Motor Speedway spokesman Fred Nation said Monday, “We are fortunate to have excellent cooperation with all appropriate federal, state and local agencies to monitor and protect the security of our fans.”

Officials estimate between 200,000 and 300,000 people from around the world will attend the Indy 500. And during the NASCAR season, thousands of people pack small towns across the country for as long as 10 days.

Monday’s FBI/Homeland Security assessment did not address health concerns. But it hammered on the ease in which terrorists could steal or falsify identification to get access to the event areas.

The assessment listed instances in the past six months when law enforcement credentials were lost or stolen in the Indianapolis area. Among those are an Indianapolis Metropolitan Police Department badge and identification which were reported stolen on December 15; a local deputy prosecutor’s badge and driver’s license reported stolen on Dec. 21; a firefighter’s badge and military ID card stolen March 3; and a Tennessee homeland security official’s ID that was stolen March 30 from a car parked in Indianapolis.

For the report, congressional staffers went to the Talladega SuperSpeedway in Alabama and Lowe’s Motor Speedway in North Carolina last October to observe how NASCAR managed security. The congressional investigators also met with members of Major League Baseball, the NCAA, the NFL and presidential nominating convention security officials in Colorado and Minnesota.

The congressional report found that infectious diseases or toxins and could inflict widespread illness at NASCAR events because of the sheer number of fans gathered in small towns that are not used to providing public services for that many people. The congressional report also notes that a naturally occurring disease, such as the flu, could cause an outbreak at a mass event. Because of this, it is important that people are up-to-date with their immunizations, the report said.

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Last September, Democrats on the committee were criticized when a committee staffer suggested that staff aides get immunizations before visiting health facilities at the two racetracks.

Taking all this into consideration, NASCAR has found a way to work with local, state and federal law enforcement as well as public health agencies to not only protect the areas where the races are held, but also to prepare to respond should there be an attack or other emergency that would produce mass casualties, the congressional investigation found.

NASCAR requires that each host facility has an emergency plan. In addition, Alabama and North Carolina deployed regional public health teams to the stadiums during NASCAR events last year.

The congressional report encourages other states and organizations to make similar security arrangements. For major events, the highest level of officials — from NASCAR executives to state homeland security advisers — should be involved in the planning, it said.

The report also calls on state and local law enforcement and emergency responders to partner with public health agencies all year round, not just during major events.