In her first TV interview since the killings, Sue Klebold apologized on behalf of her son Dylan.
The 17-year-old and his best friend Eric Harris, 18, came to school on April 20, 1999 dressed in black trenchcoats and carrying four guns. They opened fire, killing 13 people and wounding 24 in Columbine before killing themselves.
Nearly 17 years have passed since that fateful day, and Klebold said in an interview with ABC's 20/20 news show that has been living with guilt ever since.
"Yet I know that just saying I'm sorry is such an inadequate response to all this suffering. There is never a day that goes by where I don't think of the people that Dylan harmed."
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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.
"The last moments of his life were spent in violence, sadism. You know, he was cruel and hateful and -- and I have to own that."
"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 I wasn't able to stop his hurting other people. I wasn't able to stop his hurting himself. And it's very hard to live with that."
Klebold said her son was a shy, likeable kid who went to parties with friends and grew up in a loving two-parent household.
There were no signs of Dylan's murderous tendencies, Klebold said.
"I don't ever, for a moment, mean to imply that I'm not conscious of the fact that he was a killer, because I am," she said.