WASHINGTON (AP) -- A few minutes before noon next Monday, hundreds of firefighters and police officers -- including some in bright yellow "moon suits" -- will gather near an old brewery off Interstate 5 in Seattle.

It soon will seem as if a radioactive "dirty bomb" has exploded.

Around the same time, hospital officials in and near Chicago will notice a sudden increase in people complaining of flu-like symptoms. Within minutes officials will confirm the presence of a deadly biological agent.

The officials will be real, but the emergencies they are responding to will not be.

The staged events are part of a weeklong drill aimed at testing the ability of local, state and federal authorities to handle a terrorist attack.

Dubbed Topoff 2 (for top officials), the weeklong drills will be the first large-scale counterterrorism exercises since Sept. 11, 2001. They were created by the Department of Homeland Security.

Homeland Security Secretary Tom Ridge explained the drill and its purpose.

"Let me be clear: This is a simulation," Ridge said Monday at a news conference. "This is a test. This is an exercise."

Ridge repeated that several times as he and other officials tried to prepare the public - particularly those in the Seattle, Chicago and Washington areas - for the security drill, the most extensive of its type in U.S. history. The exercise will cost an estimated $16 million and involve more than 8,500 people from 100 federal, state and local agencies, the American Red Cross and the Canadian government.

"Our objective is to improve the nation's capacity to save lives in ... a terrorist event," including use of weapons of mass destruction, Ridge said.

Seattle Mayor Greg Nickels said he was not worried that the security exercise would cause panic, noting that the event has received extensive publicity locally. Newspaper ads are planned in both Seattle and Chicago, and crews can already be seen preparing for the Seattle event, which will be centered near the former Rainier Brewery building -- now home to Tully's Coffee -- just south of downtown.

"Our intent is to let people know what we're doing," Nickels said.

Officials declined to reveal details of what will happen, but participants know the Seattle event involves a mock dirty bomb -- a crude combination of conventional explosives such as TNT packed with radioactive material -- while the Chicago event involves covert release of a deadly biological agent.

A detailed, 200-page scenario has been written for the drill, which officials said will be as realistic-looking as possible. Stand-ins will portray President Bush, Vice President Cheney and even press secretary Ari Fleischer.

"There's a lot of role-playing, but I'll be playing myself," Ridge said.

Some details have been kept from the participants in order to make the exercise more realistic, said Ted Macklin, assistant director of the Office of Domestic Preparedness. The office has been planning the event since shortly after the first Topoff drill, three years ago in Denver and New Hampshire.

"We want to keep responders guessing," Macklin said.

Participants will range from governors, mayors, county executives and other elected officials to police officers, firefighters, hospital personnel and other emergency responders.

Hundreds of evaluators will watch the exercise and report their findings for later study. Whatever lessons are learned are likely to apply to earthquakes and other natural disasters, as well as terrorist attacks, officials said.

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