Sue Klebold, the mother of Columbine killer Dylan Klebold, spoke out about the 1999 killing that left 13 dead for the first time Friday in an interview with Diane Sawyer for the ABC show 20/20.

Klebold told Sawyer that following the tragic shooting that left the community of Littleton, Colorado, and the country reeling, she couldn’t stop thinking about the victims and their families.

Sue Klebold gives first interview since Columbine shooting (VIDEO) https://t.co/2YILx3O1ND pic.twitter.com/wBv5BL3vGB

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“I just remember sitting there and reading about them, all these kids and the teacher,” Klebold said.

“And I keep thinking– constantly thought how I would feel if it were the other way around and one of their children had shot mine,” she continued. “I would feel exactly the way they did. I know I would. I know I would.”

Dylan Klebold and Eric Harris opened fire on April 20, 1999, at Columbine High School, killing 12 students and one teacher. Another 24 were wounded before the killers took their own lives.

“There is never a day that goes by where I don’t think of the people that Dylan harmed,” she said.

“You used the word ‘harmed,’” Sawyer noted.

“I think it’s easier for me to say harmed than killed, and it’s still hard for me after all this time,” Klebold added. “It is very hard to live with the fact that someone you loved and raised has brutally killed people in such a horrific way.”

Diane Sawyer interviews Sue Klebold, mother of Columbine shooter Dylan Klebold, TONIGHT during 20/20 on @13WHAM ABC. pic.twitter.com/T7qXBCornJ

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Klebold told Sawyer the tragedy changed her own perspectives about why she didn’t notice anything amiss in her son.

“I think we like to believe that our love and our understanding is protective, and that ‘if anything were wrong with my kids, I would know,’ but I didn’t know,” she said. “And– it’s very hard to live with that.”

“I felt that I was a good mom… That he would, he could talk to me about anything,” Klebold added. “Part of the shock of this was that learning that what I believed and how I lived and how I parented was– an invention in my own mind. That it, it was a completely different world that he was living in.”

According to CNN, Anne Marie Hochhalter, who was paralyzed after being shot in the massacre wrote a Facebook post directed to the mother, saying “I have forgiven you and only wish you the best.”

Sue Klebold, Mother of Columbine Killer Dylan Klebold, Speaks Out https://t.co/2ALr8rAE8t pic.twitter.com/403aoNVWXE

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The interview, her first, coincides with the Feb. 15 release of Klebold’s new memoir, “A Mother’s Reckoning: Living in the Aftermath of Tragedy.”

The profits of Sue Klebold’s book will go towards research and charitable foundations focusing on mental health issues.