Oregon will be at the center of the biggest terrorism exercise ever staged nationally when TOPOFF 4 begins with a fictional "bang" from an imaginary "dirty bomb" to test the emergency response of local, state and federal agencies.

But Gov. Ted Kulongoski says it will also prepare the state for inevitable natural disasters, such as a major earthquake.

"Oregon is not immune from natural disasters and emergencies. We will keep having them," Kulongoski said. "That means government at every level must be prepared to respond effectively during a real crisis."

Short for "top officials," the fourth TOPOFF exercise since the drill was established by the Clinton administration and

Congress, will involve more than 50 local, state and federal agencies in a five-day test of their preparations and response to a staged terrorist attack.

Much of the event will be staged at Portland International Raceway along the Columbia River, but it will also spread out to other areas of the Portland metro area.

"This is a big deal for Portland, this is a big deal for Oregon," said Portland Mayor Tom Potter, who joined Kulongoski and a host of government, law enforcement and military leaders in Salem to announce the exercise.

The scenario also features simulated dirty bomb attacks in Arizona and Guam to evaluate the ability of federal agencies to manage multiple sites and will involve officials from the top levels of government, including the Defense and Homeland Security departments.

"It's going to challenge the office of Homeland Security," Kulongoski said. "That's what it's all designed to do -- to see how we react with each other."

Security experts said one of the main problems during a crisis is maintaining effective communications, which the latest drill is designed to test extensively.

"I think in all the exercises that's a huge thing, and a common theme in all of them," said Jennifer Nuzzo, a senior analyst at the Center For Biosecurity of the University Of Pittsburgh Medical Center.

One of the other difficult questions in disaster planning is how the public will react to a real emergency, and that involves mostly guesswork the drill is not designed to answer, said Gary Pearlstein, a Portland State University expert on terrorism.

"One of things we're lacking is, nobody is really educating the public on how to do the things they need to do," Pearlstein said, noting the British government has been making sure that businesses and shops have first-aid kits available.

The first TOPOFF was held in May 2000 with a simulated chemical attack in Denver and a staged biological attack in New Hampshire.

The second exercise, TOPOFF 2, was staged in Seattle and Chicago in May 2003, while the third drill, TOPOFF 3, was held in April 2005. It simulated biological attacks in New Jersey and chemical attacks in Connecticut. Canada participated in both drills, and the United Kingdom was part of the third exercise.

Details from the 2005 exercise have yet to be publicly released, prompting an inquiry from a House subcommittee on Homeland Security this week.

Kulongoski said much of the assessment of the upcoming fourth exercise will be made public, especially the role of local and state agencies. But he said it was best to keep some of the results under wraps.

"One of the harsh realities of the world today is there are people who want to do this nation harm," the governor said. "You have to understand, if we do an exercise like this and we find out a way to do things right, it's kind of foolish to tell the bad guys what we're doing."