A onetime Florida police officer has been sentenced to probation and no prison time for mistakenly shooting and killing a retired Twin Cities school librarian during a citizens academy exercise more than three years ago.

Lee Coel, 31, pleaded no contest Wednesday in a Fort Myers, Fla., courtroom to second-degree manslaughter as part of his agreement with prosecutors for the August 2016 death of Mary Knowlton, 73, during a "shoot/don't shoot" exercise.

Lee County Judge Margaret Steinbeck put the fired Punta Gorda, Fla., officer on 10 years' probation. Coel also must make restitution and has agreed not to work as a police officer. Coel could have received up to 15 years in prison.

Knowlton's husband, Gary, and son Steven attended the sentencing and spoke against the plea deal. Gary Knowlton was there when his wife of 55 years was shot during the demonstration.

"I don't believe that justice, whatever it is, will ever be done," Steven Knowlton said in court. "So why weren't other people charged? I don't understand."

Since the killing, Gary Knowlton said, "I've been working on helping my two sons heal. … We've made some pretty good progress."

When it was his turn to speak, Coel fought back tears as he told the court, "I don't know if they will ever truly forgive me because I don't know if I ever will actually ever forgive myself for what happened."

Coel was charged in February 2017 with felony manslaughter. His attorney has maintained that Coel did not know there were live rounds in the gun. An investigation determined that live ammunition had been mistakenly mixed in with blanks.

Mary and Gary Knowlton, formerly of Prior Lake, were among 35 Citizens Academy participants at police headquarters, where Mary Knowlton and another person were chosen for the exercise designed to re-create a lethal-force scenario.

She assumed the role of a police officer while Coel was decked out as a "bad guy" in a hoodie and mask with a revolver.

The family agreed to a settlement with Punta Gorda in October exceeding $2 million.

The police chief at the time, Tom Lewis, was charged with culpable negligence and was acquitted by a jury. The city fired him two months later.

Mary Knowlton was the librarian at Parkview Elementary School in Rosemount from 1988 to 2004, then was a substitute librarian at other schools in the Rosemount-Apple Valley-Eagan district through 2013. The Knowltons moved to Punta Gorda from Prior Lake, where they had lived since 1973. They have another son, William.

The Associated Press contributed to this report. Paul Walsh • 612-673-4482