A Florida police officer shot and killed a 73-year-old retired librarian in a "tragic accident" during an exercise with local residents that was meant to involve simulated lethal force, authorities said on Wednesday.

Mary Knowlton was hit by a live round fired by Officer Lee Coel on Tuesday night during a role-playing exercise at an event hosted by the Punta Gorda Police Department for about 35 members of the community, the agency said in a statement.

The married mother of two sons was a retiree from Minnesota and had signed up for the city's Citizens Academy as a show of support for law enforcement after deadly attacks on police in Dallas and Baton Rouge, Louisiana, son Steve Knowlton told NBC News.

The academy consists of a series of sessions designed to provide insight into city government and includes site visits to various agencies, according to Punta Gorda's website.

Mary Knowlton's neighbourhood had recent break-ins, and she was playing the role of an intruder when she was fatally shot, said her son, who could not immediately be reached by Reuters.

Police Chief Tom Lewis gave reporters few details about the revolver fired in the incident. It had been used in similar exercises before, and officials thought only blank rounds were available for it, Lewis said.

"We were unaware that any live ammunition was available to that officer at the time," the chief said.

Officer Coel, who has been on the force since March 2014, is on administrative leave while the Florida Department of Law Enforcement investigates. The Punta Gorda Police Department said Coel is a frequent presenter at "shoot/don't shoot" role-playing scenarios, as well as at youth and neighbourhood events.

"He's very grief-stricken," said Lewis, who added that counseling will be made available next week to others affected by the incident.

Punta Gorda is a city of about 18,000 people located on Florida's west coast, about 30 miles (50 km)north of Fort Myers.

The chief described Mary Knowlton as "a phenomenal person in this community" who attended many local events.

"Everyone involved in this accident is in a state of overwhelming shock and grief," Lewis said in an earlier statement.