Seattle ? A national bioterrorism drill for hundreds of firefighters, police and other emergency workers began Monday with a mock explosion of a radioactive “dirty bomb” in a car in a Seattle industrial lot.

Meanwhile, volunteers at Pacific Lutheran University near Tacoma, about 40 miles to the south, simulated a second, simultaneous attack. The attack involved a car bomb as well, but the scenario also called for a terrorist to run into a campus building and take hostages.

The five-day drill, combining the Seattle disaster with a mock bioterrorist attack in Chicago, is aimed at testing the readiness of local, state and federal authorities. It is the nation’s first large-scale counterterrorism exercise since the Sept. 11 terrorist attacks.

The idea, said Mayor Greg Nickels, is for regional and national agencies to see where strengths and weaknesses lie.

“When a disaster occurs, people do not call the White House,” Nickels said. “They call 911.”

The exercises, which are being spearheaded by the Homeland Security Department, 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.

On Monday afternoon, fire and police personnel in Seattle began arriving as spot fires were set throughout the industrial site. People trapped on a smashed bus cried for help. There even was a fake news crew.

In Washington, D.C., officials were running the entire drill from a hotel ballroom. About 80 federal workers from 26 agencies sat at long rows of tables, working phones and laptops and directing their agencies’ work. Computers projected maps of Seattle and Chicago onto large screens.

Seattle police officers scramble over rubble and near demolished buses during a terrorism-response exercise in Seattle. The bioterrorism drill, the most extensive in the nation's history, included hundreds of firefighters and police officers responding to the mock explosion of a radioactive dirty

Over the next few days, a number of “patients” are to show up at hospitals in the Chicago area, suffering from flu-like symptoms. In the script, a terrorist group releases a deadly plague in aerosol form.

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 Dick Cheney and even press secretary Ari Fleischer.

Some details have been kept from the participants to make the exercise more realistic.

The Office of Domestic Preparedness has been planning the event — dubbed “Topoff 2” for “top officials” — since the first such drill, “Topoff 1,” was conducted three years ago in Denver and New Hampshire.

Seattle’s mayor said he is not worried that the security exercise may cause panic, noting that the event has received extensive publicity locally.

Hundreds of evaluators will watch the exercise and report their findings for later study.