Columbine killer's mom cries as she says victims' families should be angry: I would feel the same way

The mother of Columbine killer Dylan Klebold has given her first television interview about the 1999 mass shooting.

Sue Klebold spoke to ABC News' Diane Sawyer to talk about her son and his actions as she promotes a new memoir, “A Mother’s Reckoning: Living in the Aftermath of Tragedy,” whose profits she will donate toward research and charitable foundations that focus on mental health.

“There is never a day that goes by where I don't think of the people that Dylan harmed,” she said, and Sawyer commented on her choice of verb.

“I think it's easier for me to say harmed than killed, and it's still hard for me after all this time,” Klebold 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.”

Her son was 17 when he and a friend, 18-year-old Eric Harris, opened fire April 20, 1999, at Columbine High School, killing 12 classmates and one teacher, as well as wounding 24 other people, before they took their own lives.

Klebold said she believed, as most parents do, that she would have known if something was wrong with her son -- but she said it was very hard to live with the guilt that she had failed to recognize any warning signs.

“I felt that I was a good mom ... that he would, he could talk to me about anything,” she said. “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.”

Klebold said she understands why the families of her son's victims are angry with her.

“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. I would feel exactly the way they did. I know I would. I know I would.”

The entire interview will air Friday night at 10 p.m. on "20/20."

Klebold has spoken to reporters and written in the past about her son and the massacre, but this is her first television interview.

"I wish I had known then what I know now: that it was possible for everything to seem fine with him when it was not, and that behaviors I mistook as normal for a moody teenager were actually subtle signs of psychological deterioration," Klebold wrote on the website for her book.

Watch this excerpt posted online by ABC News: