It’s been just over 10 years since Dylan Klebold and Eric Harris stepped into the halls of their high school with intentions to kill hundreds of their classmates and teachers but even a decade isn’t enough time to erase the memories of that tragic day.

Susan Klebold – mother of Dylan Klebold – has been desperately trying to find answers and insight into why her son was so deeply depressed and suicidal before the shooting. This week, for the first time since the incident, Klebold opens up to the world in an essay she wrote for Oprah Winfrey’s O Magazine.

Dylan, who was 17 at the time of the Columbine shooting, and Eric, who was 18 at the time, ended up taking the lives of 12 students and wounding 23 others before killing themselves in the library of their school.

10 years later, Susan still struggles to find peace within herself:

“For the rest of my life, I will be haunted by the horror and anguish Dylan caused,” Klebold writes in her essay. “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." 

She also offers insight on how Dylan left her home the morning of the school shooting:

"Early on April 20, I was getting dressed for work when I heard Dylan bound down the stairs and open the front door. 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."

Read the full interview in the November Issue of O, The Oprah Magazine, on newsstands now.