Here's David Muir. Reporter: Almost four years to the day after Levi Karlsen was crushed to death under a pickup truck, that insurance policy taken out just 17 days before, his father, the executor,... See More
Here's David Muir. Reporter: Almost four years to the day after Levi Karlsen was crushed to death under a pickup truck, that insurance policy taken out just 17 days before, his father, the executor, Karl Karlsen now sitting inside this interrogation room across from investigators. What killed him? The truck. How did the truck kill him? It landed on him. And I had nothing to do with the truck landing on him. Nothing. Absolutely nothing. Reporter: You haul him in and you begin asking questions. How many different stories did he have? Three. Reporter: Version one, Karlsen says he and his wife come home from a funeral and a dinner afterward to find Levi dead under the truck. But soon, version two, he was now saying that he and his wife were preparing to leave for that funeral when he discovered his son in the garage, pinned under the truck. The truck had already been fallen over. And I found him dead. And -- So this was around noon, you found, around noon or just before noon, you found him? You went out there, and the truck was rolled over on him? Yeah. And I Panicked. I don't know. I don't know. So you, you panicked in what regard? Just, I left him. Now, you, you saw, did you run over and call medical help, call 911? Did you get help? No, I didn't do anything. I, I Froze. Now, was there a phone in the garage or a cell phone? No, no, no. Or did you run back to the house and ca? 911 No. I, I went to the funeral. Reporter: So he saw his son trapped and dead under the truck and still left for the funeral? Yes. Reporter: He says that father didn't return until four hours later. During the interrogation, Karlsen asks repeatedly for his pain medication, medication he says might have altered his behavior the day Levi died. Like I say whether it's partially to do with the medicine, I don't know. I, no, I don't play that card or whatever. Reporter: And then several hours into the interrogation, Karl Karlsen appears to be on the verge of yet a third version of his story. I could never hurt him. I couldn't. You're that close, man. You're close. Come on. Let it out. Let it out. Let it out. I'll walk with you. Man, I'll walk with you. Was it just a split-second thing? I couldn't hurt him. It was an accident. I opened the truck door. Okay. And when I did, it With the wire. Basically, how it's happened, we've gone from he was dead when you walked in there to now it fell when you opened the door. I know. So take the final step. There is no more. I stepped in the truck and the Thing fell. And I was just Scared . Reporter: His son trapped by the truck and yet they still leave the house? Yes. Reporter: I mean, does that make any sense? No. I can't imagine walking away and leaving your child dying on the floor. Reporter: They were going to a funeral. Yes. Reporter: And yet many would argue he had just created one right here in his own garage. It's fair to say that. Reporter: But Karl Karlsen continued to insist he's innocent, opening up to the Syracuse "Post-standard" about his son from inside the county jail. Well, Levi, I mean, when he was younger, we had problems with him. School was hard for him. Losing his first mother was really traumatic, would be for anybody. Reporter: He talks about Levi as a young father, that he took his son back in after Levi's divorce. My wife and I took him back in. And he, I decided to stick my neck out for him and told him what his responsibilities would be. He became responsible, he had manners, he grew up. And he realized that mom and dad weren't out to make his life miserable. He got life back into him. Reporter: Karlsen, now charged with murder in the second degree, entered the Seneca county court house in December to enter his plea. And in a shocking moment of truth, he pleads guilty to killing his son Levi. Judge Dennis bender sentencing Karlsen to 15 years to life. Levi's sister Erin and her aunt Colette say they're relieved but still hopeful investigators in California will charge Karl Karlsen in the death of his first wife too. I believe Karl killed my sister. And he killed my nephew. And it's all linked to money. Reporter: And tonight, 23 years after that fire, a new investigation long after Levi's death. The California D.A. Has now announced they are charging Karlsen with the murder of his first wife Christine. And I wanna hear it from him. I wanna hear those words come out of his mouth.
This transcript has been automatically generated and may not be 100% accurate.