Two men burst into a banquet room and spray guests with a white gas. Preliminary findings indicate the substance is anthrax. Antidotes from the National Pharmaceutical Stockpile are flown in.

The surgeon general, Arizona Gov. Jane Hull, public health officials and thousands of volunteers were bracing Wednesday for one of the nation's worst nightmares: a bioterrorism attack on U.S. soil.

Tucson officials launched a three-day drill that will include mock attacks, emergency evacuations and medication airlifts to test how well the nation is prepared for a biological attack.

"It's fitting that this exercise is starting the day after the Homeland Security bill passed," Hull said. "The new reality requires a collective need to protect and defend our communities and our nation from a strange, new kind of warfare."

U.S. Surgeon General Richard Carmona said federal officials are eager to learn lessons from the exercise, one of the nation's biggest bioterror drills since the Sept. 11 attacks.

"Build the resiliency, the capability into the American public that most people don't believe is there yet," he told participants at the Tucson Convention Center.

The Tucson drill had been planned since the spring of 2000, and similar exercises will be staged across the country, Carmona said.

The simulations will test the ability of local, state and federal emergency and public health agencies to coordinate their response.

After the mock assault at the convention center, 500 participants, mainly public health officials, discussed how to respond to such an attack.

Later Wednesday, Gov. Hull was to request an aid package from the national stockpile. A 6-ton training package of equipment and mock medications will be shipped from one of 10 secure sites around the country to the Arizona Air National Guard. Local health officials will use the contents to practice immunizing about 1,000 volunteers.

In a real attack, the package would be a 50-ton emergency package containing antibiotics, antidotes, vaccines, syringes and other supplies. It would be delivered within 12 hours of the attack.