This June 22, 2014 photo provided by ABC shows Barbara Walters, left, during an interview in Los Angeles with Peter Rodger, the father of Elliot Rodger, the 22 year old who killed six people, injured 13, before taking his own life near the campus of the University of California, on May 23, in Santa Barbara, Calif. The interview will air on a special edition of "20/20" airing Friday, June 27, at 10 pm ET. (AP Photo/ABC, Rick Rowell)
LOS ANGELES — The father of the young man who killed six people and injured 13 others near the University of California, Santa Barbara, last month says it's his "duty" to help prevent future mass killings.
In a letter provided to ABC News on Friday, Peter Rodger wrote that he wants to help people recognize warning signs of mental illness within families.
He said his son, Elliot, hid his sickness from his parents, mental health professionals and law enforcement. In hindsight, the father said, he is beginning to understand there are traits, or "markers," that family members can look out for in loved ones.
"My duty now is to do as much as I can to try and stop this from happening again," the elder Rodger wrote. "It will be a long journey involving the personal choices of individuals and families, public discussions, mental health reforms, a change in the culture — you name it. My sincere hope is that I can help by telling my story."
He said he has created a website — AskForHelp.org — with resources on mental illness and a place to share stories.
"My simple message is, if in doubt about a family member, please ask for help," he wrote.
Peter Rodger also spoke to Barbara Walters in an interview that aired Friday as an ABC prime-time special.
"This is the horror story, this is the American horror story, or the world's horror story," Rodger told Walters. "It 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."
He said the realization that his son caused so much carnage is a "reverse nightmare situation."
"When you go to sleep normally you have a nightmare and you wake up and oh, everything is OK. Now, I go to sleep, I might have a nice dream, and then I wake up and slowly the truth of what happened dawns on me. And that is that my son was a mass murderer."
Rodger said at first he thought his son was a victim of the shootings in the community of Isla Vista. Only when he went on the Internet in the early-morning hours following the killings did he learn that Elliot was the main suspect.
Elliot Rodger died of an apparently self-inflicted gunshot wound after the May 23 rampage. He killed six UC Santa Barbara students and wounded 13 other people.
Rodger posted at least 22 videos on YouTube, including a final "retribution" video the night of the attack that detailed his plans and reasons for the killings. He also emailed a lengthy written manifesto to his mother, father and therapist that detailed his plans and contempt for everyone he felt were responsible for his sexual frustrations and overall miserable existence.
Elliot's mother Chin Rodger declined to be interviewed by Walters but issued a statement to ABC saying that attention should go toward helping the family and friends of the shooting victims.
Peter Rodger said it's "very hard" to mourn a son who killed so many people, explaining that he is trying to replace his anger "with love and forgiveness."
"But at the same time, I'm also haunted by the pain and suffering and the terrorizing that this sick human being did to others," Rodger said. "And I have to live with that for the rest of my life."