This file image from video posted on YouTube shows Elliot Rodger. / AP
The father of the the 22-year-old man who killed six people and injured 13 others near the University of California, Santa Barbara says he has been living a nightmare since his son's killing spree last month.
Peter Rodger tells ABC News' Barbara Walters he is overwhelmed by how much pain and suffering his son Elliot caused.
"When you go to sleep normally, you have a nightmare and you wake up and, 'Oh, everything's OK,'" Peter Rodger says in the lengthy interview scheduled for broadcast Friday as an ABC prime-time special.
"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."
Elliot, a former student, stabbed two roommates and a friend to death in an apartment near the university campus in Isla Vista, Calif., May 23 then got into his BMW and went on a shooting spree, killing two females and injuring two other people outside a sorority house.
He claimed his sixth victim outside a nearby delicatessen, before apparently turning a gun on himself. He injured a total of 13 people.
Elliot posted at least 22 videos on YouTube detailing the planned rampage and giving the reason for the killings.
He said he was a virgin and was taking revenge on young women that he felt had scorned him. He also emailed a long manifesto to his mother, father and therapist explaining his plans and motivations for the killings.
Peter Rodger, a British filmmaker, says he was shocked by his son's behavior, but is beginning to understand there are traits, or "markers," that family members can look out for in loved ones.
In a letter provided to ABC News on Friday, Rodger writes that he wants to help people recognize warning signs of mental illness within families.
"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 also 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.
Rodger has also met the father of 20-year-old Christopher Martinez, one of his son's victims.
Copyright 2015USA TODAY
Read the original story: 'My son was a mass murderer': Dad seeks to end violence