The first test of how well prepared the United States is for a terrorist attack -- one that could be far deadlier than the attacks of September 11, 2001 -- is underway in Seattle, Washington. The five-day exercise will involve actors playing the role of the president and other top U.S. officials during simulated terrorist attacks with biological and radioactive weapons.

The nationwide exercise began in Seattle, with a simulated attack with a so-called "dirty bomb," the kind of radiological device that U.S. officials have warned the terrorist network al-Qaida may be trying to acquire.

"The time to test is before such a threat arises, and this exercise will examine the combined emergency response of local, state and federal agencies," says Gary Locke, the governor of Washington state.

Actors will stand in for President Bush and other top officials, while thousands of real responders -- everyone from law enforcement to hospitals, to federal and local officials -- test their ability to react to the kinds of biological and radiological terrorism that the Department of Homeland Security's Ted Macklin warns the nation needs to be prepared for. "Some of the threats that we're finding from the adversaries in the caves of Afghanistan, it was a logical choice to select those two elements," says Mr. Macklin.

Later in the week, the exercise moves on to Chicago, where actors will begin showing up at hospitals with flu-like systems, as that city tests how well it is able to respond to a simulated biological weapons attack.

"There will be a lot of activity in both Chicago and Seattle that people will notice, and we want them to understand what's going on, we are exercising, that we are trying to push the envelope [extend the limits] in terms of our response capabilities, to be better prepared in case of a real attack at some point," says Mike Brown, who directs the Federal Emergency Management Agency.

Homeland Security officials say they want the exercise to be as real as possible, and are confident media coverage will be able to get the message out to the public that this is only a drill and not the real thing. But not all of the results of the five day exercise will be made public, so as not to tip off potential terrorists to vulnerabilities in the nation's homeland defense system.