Mother of Columbine Shooter Speaks Out Nearly 17 Years After Massacre: Not a Day Goes by That 'I Don't Think of the People That Dylan Harmed'

For nearly two decades, Sue Klebold has tried to wrap her head around why her son 17-year-old Dylan, along with Eric Harris, 18, killed 13 people in a mass shooting at Columbine High School in 1999.

"I just remember sitting there and reading about them, all these kids and the teacher," Klebold said an exclusive interview with ABC News' Diane Sawyer in an upcoming special edition of 20/20, which airs Friday at 10 p.m.

"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. I would feel exactly the way they did. I know I would. I know I would."

In addition to killing 13 people at the Colorado school, Dylan and Harris opened fire on the Colorado school, injured 24 more before taking their own lives.

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

She said she wonders where she went wrong as a parent.

"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.

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

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She said she was shocked to learn that her view of her parenting was different from what her son may have seen.

"It was a completely different world that he was living in," she said.

The special edition of 20/20 on Friday at 10 p.m. on ABC.