VANCOUVER - A terrorist of uncertain allegiances is in charge of an artificial snow-making machine during the 2010 Winter Olympics. He mixes the snow with radioactive material and spews the concoction across a crowded field.

Although the machine is turned off after a few minutes, people in the crowd start coughing. Some feel nauseated and have difficulty breathing. One person collapses. As word spreads about the source of the problem, dozens of people race from the field to a nearby hospital. Hundreds who were not exposed and have nothing to worry about flood the emergency room, demanding to be tested.

That's one of the nightmare scenarios confronting security and government agencies this week as they test their responses to possible terrorist attacks that may disrupt the Games in Vancouver and Whistler in February.

The highly publicized, five-day exercise is intended to give the public confidence that security forces are prepared without giving any ideas to the bad guys, John Oakley, a senior manager with Public Safety Canada, said Tuesday. As he spoke, emergency crews and "victims" were acting out the mock terrorist attack in a field adjacent to now-vacant Steveston High School in Richmond, 10 minutes from the Olympic Oval skating rink.

Neither the terrorist scenarios nor the response procedures for security forces and health and government officials are confidential, Mr. Oakley said in an interview. "All this is open-source information," he said.

Officials involved in the exercise also dismissed a concern that publicity about the exercise could spread fear and discourage people from attending the Winter Games. "People will be happy we are prepared to respond to any sort of emergency," said Ted Townsend, a senior manager with the City of Richmond.

The exercise, however, may discourage the terrorists from showing up, said Michael Byers, the Canadian research chair in international law and politics at the University of British Columbia. The risk of a so-called dirty bomb at the Olympics, dispersing radioactive material and stirring widespread panic, is extremely low, he said.

"I don't exactly see why terrorists would choose to penetrate Olympic security to detonate a bomb when in normal circumstances in any big city such a bomb would create a comparable level of dislocation and draw a comparable level of media attention," Prof. Byers said.

"But by focusing some attention onto the risk, the [2010 Olympic] integrated security unit is probably providing a very effective deterrent against the possibility."

The exercise began Monday with a girlfriend of a suspected member of an unidentified terrorist group calling police. A search of the boyfriend's house turned up a laboratory with chemical and radioactive material. The police subsequently received a call from terrorists threatening to disseminate radiological isotopes from devices in up to 20 different locations, including a shopping mall filled with hundreds of people.

Tuesday morning, the exercise focused on the impact of a chemical attack on a pep rally in support of one of Canada's hockey teams. Further developments in the scenario will be revealed later this week, when emergency crews will be tested in a mock incident in downtown Vancouver.