Peter Rodger, the father of the man who went on a rampage, killing six people and wounding 13 more in Isla Vista., Calif., said his son had never shown any violent tendencies and he never thought his son "could hurt a flea."
"We didn't see this coming at all," Rodger, a photographer and Hollywood movie director, told ABC News' Barbara Walters in an exclusive interview that will air in full Friday in a special edition of "20/20."
"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.
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"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."
On the night of May 23, Elliot Rodger, 22, began his spree by fatally stabbing his two roommates and their visiting friend in a shared apartment near the campus of the University of California, Santa Barbara. He then drove around the Isla Vista community, shooting random victims and even ramming his black BMW into students.
He fatally shot himself in the head after a gunfight with police.
Rodger had had trouble fitting in socially and struggled with loneliness. In videos posted to YouTube he had talked about hating girls who were "repulsed" by him. Rodger posted a YouTube video shortly before the attack in which he outlined his retribution, and also sent a manifesto to friends and family in which he again expressed his frustration with women and couples and described what he called his "Day of Retribution."
Survivors of the attacks said Rodger smiled before pulling the trigger. His father sighed as he thought about the sorrow his son had left behind.
"Every night I go to sleep, I wake up and I think of those young men and young women that have died and are injured and were terrorized when my son did that," Peter Rodger said. "My son caused so much pain and suffering for so many families."
Watch the full interview Friday at 10 p.m. ET / 9 p.m. CT on "20/20" on ABC.
Father of Santa Barbara Shooter 'Didn't See This Coming'