In an essay in the November issue of O magazine, on newsstands today, Klebold describes the shock and horror she experienced on the day of the shooting, as well as her personal journey to comprehend her son's actions and move forward.
On April 20, 1999, 17-year-old Dylan Klebold and fellow student Eric Harris, who was 18, opened fire inside their high school, killing 13 and injuring more than 20 before turning their weapons on themselves.
"For the rest of my life, I will be haunted by the horror and anguish Dylan caused," Klebold wrote. "I cannot look at a child in a grocery store or on the street without thinking about how my son's schoolmates spent the last moments of their lives. Dylan changed everything I believed about myself, about God, about family and about love."
O magazine editor at large Gayle King told "Good Morning America" today that it wasn't the anniversary of the shooting that inspired Dylan's mother to share her story, but instead, "she said it's really taken 10 years for her to process."
"She still feels unspeakable grief," King said. "As a parent, you think, how could you not know?"
Klebold was at work when she got a phone call from her husband telling her that there had been a shooting at the high school and Dylan was unaccounted for.
In her essay, Klebold describes an exchange she had with Dylan on the morning of the massacre and says she is haunted by not having seen him face-to-face that day.
"Early on April 20, I was getting dressed for work when I heard Dylan bound down the stairs and open the front door," she wrote. "Wondering why he was in such a hurry when he could have slept another 20 minutes, I poked my head out of the bedroom. 'Dyl?' All he said was 'Bye.' The front door slammed, and his car sped down the driveway.
"His voice had sounded sharp. I figured he was mad because he'd had to get up early to give someone a lift to class. I had no idea that I had just heard his voice for the last time."
King said Klebold "replays that scene a million times in her head."
In the decade since the Columbine massacre, the nation has learned that Dylan, who was in a program for highly intellectual students and was his father's loyal chess partner, had grown increasingly shy in his teenage years. He was bullied and turned inward to journals that describe a depressed teen who hated life.
"I'm an outcast," he wrote in one entry, "and everyone is conspiring against me."
He also wrote about events like the Oklahoma City bombing, and wanting to outdo those events.
"From the writings Dylan left behind, criminal psychologists have concluded that he was depressed and suicidal," Susan Klebold wrote in her essay. "When I first saw copied pages of these writings, they broke my heart. I'd had no inkling of the battle Dylan was waging in his mind."
King said Klebold "believes that her son had not been diagnosed with depression, that he had a mental illness that she was not aware of. She also doesn't believe that if he wasn't suicidal, that these murders would have happened."
Klebold described Dylan as a joyful and intellectually adventurous child who changed dramatically after he entered junior high school, becoming quiet, unmotivated academically and irritable.