Tonight, a "20/20" exclusive, about something that happened this very hour two months ago. And the father of the young killer speaking out only to Barbara Walters. Tonight, he's back on a special... See More
Tonight, a "20/20" exclusive, about something that happened this very hour two months ago. And the father of the young killer speaking out only to Barbara Walters. Tonight, he's back on a special assignment. Well, tonight, this is special. A milestone, the first time that a parent of a mass murderer has ever spoken on television. He's doing this to show other parents the warning signs he missed hopefully so that no one has to go through the night that changed his life and so many others, forever. On a quiet memorial day weekend, peter Rodger and his wife were hurtling down the highway from Los Angeles to Santa Barbara. It was a one hundred mile journey of terror. I have to save my son. I have to get to Santa Barbara now. Reporter: Peter Rodger had just seen a disturbing video that his son, Elliot, had posted on youtube. I hate all of you, and I can't wait to give you exactly what you deserve. Utter annihilation. What went through your mind? I can only describe like -- like a really dark force of horrible energy that hit me. What were your fears for him? I felt for his -- his life. I didn't know what -- what he was doing. And then suddenly he's saying he's going to do other things to other people. I was confused. I just wanted to go and find him and -- and talk to him, do something. We were driving up there not knowing what to do trying to get information. Reporter: Peter Rodger was on a mission to prevent a massacre. But halfway to his destination, suddenly he heard the reports. Breaking news right now out of Isla vista where we are getting reports of a shooting. There are unconfirmed reports that there may be as many as three victims. Possibly a fatality. The death toll could be climbing. Shots fired. Did you know that this was your son? The shooter? No. God -- good lord, no. We were just driving up there in absolute fear for him and -- and confusion. Witnesses say a BMW with two people inside had people open fire and also run over people. I was hearing, you know, reports of a black BMW, and I was going, "Oh, no, no, no. Police were saying, we don't have any information for you. Reporter: The driver of that BMW was his 22-year-old son, Elliot. Santa Barbara sheriff bill brown is calling the mass shooting the work of a madman but says at this time the threat appears to be over and the suspect is dead. Reporter: Seven people died that night, including the shooter, Elliot Rodger. 13 others were hospitalized. For peter Rodger, those memories are a waking nightmare. How are you? 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. When you wake up in the morning, what do you think? It's like a reverse nightmare situation. When you go to sleep normally, you have a nightmare and you wake up and, "Oh, everything's okay." Yeah. 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 -- my son was a mass murderer. And then I think about the victims and I think about what he did. And I try to process it. Reporter: But who was the young man behind the rampage? And why did he do it? Hi, Elliot Rodger here. Were there clues in the disturbing jigsaw of videos he left behind? And what would his 137 page so called "Manifesto" reveal about the private agony of a psychotic mind? Its title, "My twisted world." This is the story of how I, Elliot Rodger, came to be. It is a story of a war against cruel injustice. This tragedy did not have to happen, but humanity forced my hand. Reporter: Elliot Rodger was born in London and was brought to America at the age of five. His malaysian-born mother, Li chin, was a unit nurse on the movie "Indiana Jones and the last crusade." His father directed television commercials and worked on the hit film "Hunger games." In the beginning, Elliot had a seemingly Normal childhood. He was adorable. And he would laugh so much that sometimes we were worried he would choke. It was really wondrous, those first four, five years of his life. It was wondrous. You loved Elliot. I loved Elliot. Elliot loved you. I -- yeah. Reporter: But at the age of 7, Elliot's parents got divorced. One year later, his father had remarried. This time to Moroccan born soumaya akaaboune, an actress who appeared in the Hollywood blockbuster, "Green zone." It was during those years that 7-year-old Elliot began his long treatment in therapy. Why did you think he needed it? When we realized he had social issues, we brought therapists in to try and help him integrate into society. Did he have trouble with other children, with making friends -- Yes. He had very few friends in elementary school. Because he was a very, very, very, very shy -- shy boy. And we just felt that that was his issue. Reporter: On the surface, Elliot seemed destined for a life of success and privilege. Peter Rodger's work gave his son access to Hollywood's elite. From an early age, it was a world he longed to join. My little 9-year-old self realized that there were hierarchies, that some people were better than others. Jealousy and envy, those are two feelings that would dominate my entire life and bring me immense pain. Reporter: By any standard, the Rodger family was living "The good life." But those tantalizing glimpses into Hollywood's inner circle would always leave him resentful and wanting more. Did he blame you for the fact that he wasn't a rich kid? He just had illusions of grandeur. Even if he couldn't make it himself he wanted to feel in a position where we would somehow provide it for him. Reporter: The relationship Elliot shared with his father had always been complicated. Peter Elliot was the mount Everest his son could never reach. You are a very good-looking man. Did he realize that? Did that please him? Was he -- He was jealous of me and -- the way that I looked. Reporter: The shy, young boy had always harbored fear and self doubt. His parents moved him from school to school. Hoping, but never succeeding, to help Elliot fit in. There was some question about whether Elliot was autistic. He was never formally diagnosed. There was a suggestion that he might have been high-functioning asperger's. And you know, these were because of traits. For instance, there was a certain O.C.D. About him. There was a certain, you know, like he'd always put his plate in the same place. He liked to wear the same clothes every day. Elliot really was -- a very high-functioning human being. If he were sitting here right now, you would think, "What a polite boy he was." But yet, he had this thing going on inside of him. Reporter: What his parents didn't know was that Elliot, even at the age of 10, was hiding a secret that would someday transform a shy young child into a cold blooded killer. This is the horror story. 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.
This transcript has been automatically generated and may not be 100% accurate.