This is what it's like to be the mother of a mass murderer.

It was April 20, 1999 when, clad in black trenchcoats and wielding four guns, Dylan Klebold and Eric Harris walked into Columbine High School and killed 12 students and one teacher. It came after a foiled attempt to blow up hundreds of their classmates and ended in the duo taking their own lives.

So many experienced unfathomable loss. The effects of the tragedy were felt far beyond the confines of the school gates, with an entire country mourning the loss of innocent lives and a set of gun laws that left them so vulnerable.

For Sue Klebold, mother of Dylan, it was a tragedy she was not prepared for nor was she expecting.

In a TED Talk, she has revealed how much she has struggled to navigate her son’s legacy and the fact she raised a mass-murderer.

“There’s no way to asses the magnitude of a tragedy like Columbine, especially when it can be a blueprint for other shooters who went on to commit atrocities of their own,” she told the audience.

“It’s taken my years to try and accept my son’s legacy. The cruel behaviour that defined the end of his life showed me he was a completely different person from the one I knew.”

Klebold went on to lament the fact she had no inkling her son was capable of such violence.

“Afterwards, people would ask: How could you not know? What kind of a mother were you? I still ask myself those same questions. Before the shooting, I still thought of myself as a good mum. Helping my children become caring, healthy, responsible adults was the most important role in my life. But the tragedy convinced me that I failed as a parent.

"Aside from his father, I was the one person who loved and knew Dylan the most in the world. If anyone knew what was happening, it should've been me, right? But I didn't know."

Acknowledging she tried to "comb through memories" for years after the tragedy to try and see the warning signs, Klebold said she cannot provide answers or solutions. She can only tell the story of what she has learned.

"When I walk into a room like this, I never know if someone has experienced loss because of what my son did. I feel the need to acknowledge the suffering caused by a member of my family, who isn't here to do it for himself.

"From the bottom of my heart: I'm sorry, if my son has caused you pain."

Today, Klebold explores the intersection between mental health and violence, passionate about advancing mental health awareness and intervention.

You can watch her full talk here: