PUNTA GORDA, Fla. -- The officer who shot and killed a 73-year-old retired librarian during a police "shoot/don't shoot" demonstration in Florida was accused of using excessive force with his police dog and resigned from another police agency in 2013 for failing to satisfactorily complete a field training program.

Officer Lee Coel, 28, was put on administrative leave as the Florida Department of Law Enforcement investigates why real ammunition was used by mistake at an event designed to bring police and the public together in the small Gulf Coast city of Punta Gorda.

Chief Tom Lewis said Thursday he is taking "full responsibility" for the shooting, which he witnessed on Monday night. He expected the state agency to finish its investigation in about two to four weeks.

Coel has worked for the department since 2014 and frequently gave department presentations and tours, "specifically role-playing in these shoot/don't shoot scenarios," police spokeswoman Lt. Katie Heck said.

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The department did not make Coel available for comment Wednesday. A woman who answered a phone listed for Coel said she didn't want to talk and hung up.

The chief said the officer is grief-stricken and other officers have been assigned to him to make sure he's psychologically stable.

On Tuesday night, Mary Knowlton, a well-known community volunteer, assumed the role of the officer during the "shoot/don't shoot" exercise, Heck said.

During such an exercise, the citizen "assumes the role of the officer, and is confronted with situations in which they must make a decision about whether to use force on the role-player," Heck said. "The situations escalate quickly, forcing fast decisions. Historically, it fosters a better understanding for what officers face during an intense situation, and leads to informative dialogue between the community and officers who act as role players."

Both the citizen and the officer have a firearm during these scenarios.

But the guns are either supposed to be loaded with blanks or "simunition guns," which are real-looking weapons that fire a non-lethal projectile with reduced force. Knowlton was mistakenly struck with a live round.

The police chief said his department has been doing the exercise for about two years.

Mary Knowlton attended the class with her husband and it was supposed to be "a fun night," her son said Wednesday.

Coel left the Miramar Police Department after 14 months of service in the Broward County agency. Tania Rues, Miramar police spokeswoman, said Coel resigned, but could not comment on the reasons why. Coel wrote a resignation letter saying he was resigning for "personal reasons;" the News-Press reported that he failed to complete an agency field training program.

A Punta Gorda lawyer said Wednesday that Coel shouldn't have been on the Punta Gorda force. Scott Weinberg is representing a man who said he was mauled by Coel's K-9 during an arrest in November.

Coel was accused of letting his K-9 attack Richard Shumacher for nearly two minutes after he stopped him for riding his bike without a light, CBS News correspondent David Begnaud reports.

Weinberg took the man's case in June, and that's when he viewed Coel's dashcam video of the arrest and informed local media about the case.

"I told everyone that this officer was dangerous and he needed to be fired," said Weinberg. "If he had been fired like he should have been when he ordered that dog to maul my client for a minute and 47 seconds, then this wouldn't have happened."

Punta Gorda officials aren't saying how a gun with a live round came to be used at Tuesday evening's demonstration, noting blank rounds are typically used in such classes.

The victim's son, Steve Knowlton, said in an interview at his parents' home that, on his mother's behalf, he was forgiving the officer who fired.

Mary Knowlton moved to Florida after living for years in the Twin Cities area of Minnesota. Books and magazines lay scattered on tables of the home she shared in Florida with Gary, her husband of 55 years. The couple split their time between Minnesota and the small Gulf Coast community. She had two sons.

"There's too much hate in this world, in America, we always feel like we need revenge and it doesn't solve anything," Steve Knowlton said. "I obviously can't say it's easy to forgive, but it needs to be done. She's watching me now."