The most extensive terrorism-response drill in U.S. history began Monday in Seattle, Washington. The goal is to see how local, state, and federal agencies respond to the type of emergency that would follow a major terrorist attack. Monday's scenario dealt with a radioactive, so-called "dirty bomb." The simulated attack began with two explosions just south of downtown Seattle, and the release of acrid smoke. Police, firefighters, and medical workers converged on the area, and sifted through debris.

Eric Holdeman is head of King County's Office of Emergency Management. He says participants in the drill knew beforehand that the city would be the target. But beyond that, they didn't know what to expect.

"We really have kept how the whole event will unfold a secret from those players who are participating, because there's no value in doing an exercise, if everybody knows what is going to happen," he said. "So, we want that surprise element in there. So, it is a great training opportunity."

The five-day exercise later moves to Chicago, with the simulated release of a deadly biological agent. Mr. Holdeman says it is not a test to show that agencies are ready for an attack, but to look for areas where they are not prepared.

"The more mistakes we make here, the happier I will be," said Mr. Holdeman. "Because, you either make them in real life, and then people suffer the consequences, or we can make them here, and say, 'oops, boy we can't do that again. This is broke; we got [have] to fix it."

The exercise, created by the Department of Homeland Security, is estimated to cost $16 million. It involves more than 8,500 people from federal, state and local agencies, the American Red Cross and the Canadian government.