Losing a child is one of the most painful things a parent could have to endure – when your child turns out to be a mass murderer, how do you cope?

Elliot Rodger’s father, Peter, will appear in an exclusive interview with abc’s Barbara Walters to be aired in the USA today.

The bereaved dad said his son had never shown violent tendencies and he had no idea he was capable of such an act.

“This is the American horror story, or the world’s horror story, is when you have somebody who on the outside is one thing, and on the inside is something completely different. And you don’t see it.”

Rodger told Walters that he’s now living a “reverse nightmare” as a result of his son’s actions.

“When you go to sleep normally, you have a nightmare and you wake up and, ‘Oh, everything’s OK,’” he said. “Now I go to sleep, I might have a nice dream. And then I wake up and it’s, slowly, the truth of what happened dawns on me. And you know, that is that my son was a mass murderer.”

In the killing spree Elliot killed 6 people and injured 13, some of whom he shot and other he rammed with his car. He fatally shot himself and crashed his car to end the rampage.

Find more info and a preview of the interview here on ABC News.