As of 9 a.m. Tuesday, a lethal cloak of radiation will cover much of downtown Portland.

It's all make-believe, of course. But this week the city will be ground zero for the largest counterterrorism exercise in U.S. history.

Homeland Security Secretary Michael Chertoff will be in town to personally oversee the exercise, with the help of Gov. Ted Kulongoski and Mayor Tom Potter. In all, more than 15,000 people from 275 organizations will be involved. Every emergency agency in the Portland area, along with 14 hospitals and David Douglas High School, will play parts in an event dubbed "Topoff 4."

Six years after 9/11, with the nation fighting wars on two foreign fronts, skeptics say such exercises only inflame the public. But federal, state and local officials say they need the complex drills to test their ability to work together during large-scale catastrophes. The history of Topoff shows those tests don't usually go well.

"We know that the world will be watching," said Chip Terhune, Kulongoski's chief of staff.

What the world will see is simulated dirty bombs going off in Portland, Phoenix, Ariz., and the U.S. territory of Guam, as federal officials test their ability to react to simultaneous terrorist attacks designed to stretch emergency resources.

Portland's fake bomb is staged to detonate on the Steel Bridge. But a grassy patch covered with banged-up cars, debris and a mock-up of the bridge near Portland International Raceway will stand in for downtown. The U.S. Bureau of Alcohol, Tobacco and Firearms and Explosives will discharge an explosive at the site -- mostly for TV cameras.

Firefighters and police arriving at the site Tuesday will encounter lingering smoke and chaos, with as many as 350 actors in makeup playing injured and contaminated victims and, possibly, terrorists. As real journalists from around the world watch from a distance, fake reporters with the "Virtual News Network" will swarm the scene.

And the role-playing won't stop there. Chertoff, Kulongoski, Potter and Multnomah County Chairman Ted Wheeler also will behave as if the blast is the real thing.

Only the basic scenario is spelled out ahead of time, as other cities that have scrambled to keep up during past exercises have learned. State and city officials say they expect to be thrown a lot of curveballs.

"When you have an opportunity to test how far your entire system can be stressed," said Carmen Merlo, director of Portland's Office of Emergency Management, "you take it."

First drill in 2000

In 1998, alarmed by a deadly sarin gas attack on the Tokyo subway, Congress called for a national test of terrorism-preparedness every two years. Topoff, short for "top officials," was the program born of that mandate. It costs about $25 million a year.

The first drill in 2000, involving a covert attack using the pneumonic plague microbe on the Denver Performing Arts Center, exposed a long and embarrassing list of vulnerabilities.

The city's "Ten Days in May" effort was shut down after only 4 1/2 days, as the faux plague spread out of control. The national emergency-management system collapsed before state and city health officials even devised a plan to distribute antibiotics.

Greg Moser, a University of Denver professor of homeland security and former counterterrorism specialist for the Colorado Office of Emergency management, said nearly everyone walked away frustrated and a bit shaken.

Three years later, Seattle officials went into "Topoff 2" fully expecting things to break down. "We figured that was the only way to really prepare and learn," said Seattle police Capt. Mike Meehan, the drill's incident commander.

The exercise started to go haywire almost immediately. Everyone knew the scenario ahead of time: a dirty bomb in the heart of the city. Still, the first police and firefighters arrived without gas masks and radiation gear, forcing them to pull back.

Then a scripted FBI warning about a sniper and a possible second bomb left first responders confused, slowing their ability to get blast victims into ambulances. Because of conflicting data from different agencies about the contamination zone, Mayor Greg Nickels took an hour to order thousands to "shelter in place." Meanwhile, video conferencing equipment broke down, and officials realized they had no common emergency channel to relay urgent messages.

"There were literally hundreds of lessons learned," Meehan said. Many of the flaws were subsequently fixed. The city's hazardous-materials and SWAT teams, for example, came up with a strategy to deal with the threat of snipers at an emergency scene. But when it comes to communications and crowd control, he said, "there will always be problems."

Even as "Topoff 4" gears up, details from the most recent exercise, a 2005 simulation in Connecticut and New Jersey, have yet to be released. Last week, Homeland Security officials held a closed-door briefing with members of Congress who had demanded to see "sensitive information" presumed to be holding up release of the after-action report.

Oregon wanted exercise

Oregon was among 14 states to apply for the opportunity to host this year's exercise. In a 2004 letter to federal officials, Kulongoski said Portland has something other large cities with seaports and international airports lacked: "11 bridges that cross two rivers and support two interstate highways."

Portland's bridges add a new wrinkle to the challenge, said Bill McNally, national drill director for Homeland Security.

The state's application shows Oregon wanted the exercise, in part, as a way to acquire scarce resources. Large-scale drills are essential to preparedness, but largely unaffordable for state and local agencies. Merlo, Portland's emergency management director, said a $2 million federal grant will cover the city's costs.

Kulongoski has said the exercise is not only about terrorism, but about preparing for an earthquake or other disaster at every level of government.

Protesters may be at drill

Not everyone thinks it's just a drill. Conspiracy theorists have for months filled Internet forums and the ears of officials with their belief that President Bush will use the exercise as a front to declare martial law, then declare an eventual attack on Iran. Police expect real protesters to target the week's events.

Some, like 70-year-old Arlene Miller of Portland, worry the Bush administration has ordered the exercise to "go live," detonating a real dirty bomb and blaming it on terrorists. "I'm scared to death," said Miller, a retired Bonneville Power Administration employee.

When alarmists call the Oregon National Guard headquarters in Salem, officials are quick to tell them fewer than 40 soldiers will participate in the drill. In fact, the public won't notice most elements of the exercise at all, planners say.

Portland officials say they have received about 20 calls in the past month. "We try to make it clear that it's not a real radiological bomb," Merlo said. From there, callers are sent to 2-1-1, the city's Topoff information line.

When the "live action" portion concludes Wednesday night, the exercise will move to boardrooms and paper through Friday, concentrating on questions bound to emerge in the aftermath of a real dirty bomb.

How do you convince Portlanders it's OK to go back outside? What if cargo ships refuse to dock in an irradiated city? Would tourists choose to go somewhere else? Would property values collapse, prompting an exodus?

"In reality," Merlo said, "I think that's the part that's going to be the most interesting piece: How do we put the state back together?"

-Joseph Rose