Family members of the 73-year-old woman who was accidentally shot dead by a Florida cop during a civilian police academy course are grappling with how such a “senseless” tragedy could have happened, the woman’s son said.

Mary Knowlton, a longtime librarian, was killed Tuesday night when a role-playing exercise at the Punta Gorda, Fla., police station went horribly wrong, authorities said. Knowlton was randomly selected to be a participant in a scenario meant to teach students when to shoot and when not to shoot during lethal force situations, Punta Gorda Police Chief Tom Lewis said. The mother of two was “mistakenly” and fatally shot by an officer in what officials are calling a “horrible accident.”

“I know the police officer didn’t intentionally do it, but it just feels like our souls got ripped out of our chest,” her son, Steve Knowlton, told TIME on Wednesday. “It just turned everything upside down.”

“She was a rock for her family,” he added. “She just always kept us pretty strong, and we’re pretty lost right now. I just don’t understand how this could happen.”

It was Mary Knowlton’s love for learning and helping out in the community that brought her to the police station Tuesday night, her son said, adding that his mom was “excited” to get more involved and support local law enforcement. Her husband went with her.

The police chief told reporters that Mary Knowlton and another volunteer were chosen to participate in the lethal force simulation. Her son said several people in the class were stunned when the officer’s gun fired and Mary Knowlton fell to the floor. “She goes down and no one knows if she’s pretending. Then they flipped her over, and she had a big hole in her chest and a bullet in her shoulder,” said Steve Knowlton, 50.

The woman’s husband of 55 years, Gary Knowlton, was “just in shock” as officials rushed to give her CPR before taking her to a hospital. He called his son to break the news.

“That moment has changed my life forever now,” Steve Knowlton said. “My dad witnessed a horrible thing. This is the kind of thing some people don’t come back from. But we’re going to lift each other up.”

Mary Knowlton taught her sons never to “harbor hate,” Steve Knowlton said. The son said he goes “back and forth” with his ill feelings toward the officer in the incident, who has not yet been named and who has been placed on administrative leave.

“I think it’s just human,” Steve Knowlton said. “Life sends you lessons, and it’s really hard to figure this one out. So senseless. Such a wonderful life taken, so senselessly.”

He broke down in tears as he remembered his mother, an active volunteer who played an integral role in getting a public library set up in Prior Lake, Minn., where her sons grew up. “She was one of those iconic type figures,” he said, adding that his mom excelled as a scholar and an athlete when she was young and was held back by restrictions on women during the time. “If she would have been born later, she would probably be running our country,” he said.

Mary Knowlton always had a book in her hand, and she passed along novels to loved ones if she thought it could touch their lives. When Steve Knowlton was battling alcoholism years ago, she gave him Mitch Albom’s book, For One More Day, which tells the story of a troubled man who gets a chance to spend one more day with his mother who died eight years prior.

“After it was over, he accepted it and he knew she was at peace and she was happy,” Steve Knowlton said about the book, which he now plans to read again. “I’d give anything to have one more day to tell her how much she meant, how much of an impact she had on my life.”

“I would give anything to hear her clap again at a basketball game,” he said. “I know someday I’ll see her again. I know everything will be right. You just never know how much you appreciate somebody until they’re gone.”

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