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The mother of Columbine High School shooter Dylan Klebold says she didn't know anything was wrong with her son before the 1999 attack, and that she thinks about the victims and their families every day.

In an interview that aired on "20/20" late Friday, Sue Klebold told ABC News' Diane Sawyer that before the attack she considered herself a parent who would have known something was wrong.

"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, and it's very hard to live with that," she said.

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Dylan Klebold is shown in a 1998 yearbook photo from Columbine High School in Littleton, Colo. AP

"I felt that I was a good mom ... That he would, he could talk to me about anything," she continued. "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."

Dylan Klebold and Eric Harris opened fire at the suburban Denver school on April 20, 1999, killing 12 students and a teacher before killing themselves. Another 24 people were injured in the attack.

"There is never a day that goes by where I don't think of the people that Dylan harmed," Sue Klebold said, adding that, "It is very hard to live with the fact that someone you loved and raised has brutally killed people in such a horrific way."

Miniature crosses are displayed to commemorate the ten-year anniversary of the Columbine High School shootings at Clement Park April 20, 2009 in Littleton, Colorado. Marc Piscotty / Getty Images

The interview coincides with the release of Sue Klebold's memoir, "A Mother's Reckoning: Living in the Aftermath of Tragedy," which will go on sale Monday.

She has said the Sandy Hook shootings in 2012 helped convince her to share her story, and that she is donating any profits from the book to mental health charities and research.