A state investigation following the accidental death of a 73-year-old retired librarian during a police "shoot/don't shoot" demonstration in Florida is expected to take two to four weeks, the chief of the Punta Gorda Police Department said Thursday.

"Something went terribly wrong," Chief Tom Lewis told a news conference, noting the department had safely conducted the community demonstrations over the last two years.

Police earlier said that the shooting at the drill, which was supposed to involve the use of blank ammunition, was accidental.

Officer Lee Coel, 28, has been put on administrative leave as the state of Florida investigates why real ammunition was used by mistake at the event designed to bring police and the public together.

Lewis said he is taking "full responsibility" for the shooting death of Mary Knowlton, and that the department was working to provide grief assistance to the family and the community.

The department intends to be "completely transparent" as the Florida Department of Law Enforcement conducts its investigation, the chief said.

On Wednesday, he had said police were "unaware that any live ammunition was available to the officer. The officer involved is grief stricken. We've got officers assigned to him to make sure he's psychologically stable."

Guns supposed to be loaded with blanks

Knowlton, a well-known community volunteer, was killed while assuming the role of the officer during the "shoot/don't shoot" exercise on Tuesday night.

During such an exercise, the citizen "is confronted with situations in which they must make a decision about whether to use force on the role player," said Lt. Katie Heck, spokeswoman for the Punta Gorda Police Department. "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."

Steve Knowlton on Wednesday said at his parents' home in Punta Gorda, Fla., that he has forgiven the officer involved in the 'shoot/don't shoot' demo death of his mother, Mary Knowlton. (Chris O'Meara/Associated Press)

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" — real-looking weapons that fire a non-lethal projectile with reduced force. But Knowlton was mistakenly struck with a live round, officials said.

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

Officer previously accused of excessive force

During Thursday's news conference, the police chief did not address media coverage questioning Coel's record.

Coel had previously been accused of using excessive force with his police dog and resigned from another police agency in 2013 for failing to satisfactorily complete an agency field training program.

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

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

Coel left the Miramar Police Department after 14 months of service in the Broward County, Fla., agency. Tania Rues, Miramar police spokeswoman, said Coel resigned, but she 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. Weinberg took the man's case in June, which is 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, who didn't identify his client. "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.

Steve Knowlton, one of two children of Mary Knowlton, said in an earlier interview at his parents' home that on his mother's behalf, he was forgiving the officer who fired.

She had 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.

"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."