Mary Knowlton knew that the key to enjoying retirement in Punta Gorda, Fla., was to stay active, her friends say, and they had no idea how she kept so busy. When she signed up for a citizens academy held by the local police department, she and her husband were among 35 people hoping to learn more about law enforcement tactics.

During the training on Tuesday, which drew community leaders, business employees and even the chief of the Punta Gorda Police Department, Ms. Knowlton was selected to participate in a “shoot/don’t shoot” role-play exercise. But the session went terribly wrong.

Ms. Knowlton, 73, was accidentally shot and killed by a police officer, according to the authorities.

The police chief, Thomas P. Lewis, said at a news conference on Wednesday that Ms. Knowlton had been killed by an officer who fired a revolver that had been used before in the role-play exercise intended to teach people how to make decisions when confronted with lethal force.