did. Here now, David Muir. One family will never be together again. A beloved son gone, crushed under a $5,000-pound truck. And when investigators start digging, a shocking discovery. Was there a... See More
did. Here now, David Muir. One family will never be together again. A beloved son gone, crushed under a $5,000-pound truck. And when investigators start digging, a shocking discovery. Was there a deadly pattern after all? And tonight, a exclusive development. The daughter that says, I know what you did. And she has authorities in a second state listening, too. Reporter: It was a cold November in upstate New York, those autumn leaves vanishing in the wind and a young father about to suddenly vanish too. It wasn't unusual to find 23-year-old Levi Karlsen tinkering away in the garage under a pickup truck. He was very good working on cars. He's very mechanical. Reporter: And on one of those crisp fall days back in November of 2008, that's exactly where his father, Karl, and stepmother, Cindy, found their beloved son. Only this time, Levi wasn't working under the truck. He was trapped under it. And this was their desperate plea for help. 911. I think I need an ambulance. What's going on? The truck fell on my stepson. The truck fell on your stepson? We just got home, and I don't think he's alive. Reporter: Levi stood little chance against the three tons of metal. That pickup truck that had been propped up had come crashing down. Karl, they wanna start cpr. Do you know cpr? His chest is crushed. His chest is crushed. His chest is crushed? Oh, my god. I don't know how long he's been in there. We've been gone since noon. Okay. Oh, my god. Reporter: You can hear the cries on the 911 tape. An unthinkable tragedy for two parents who'd just found their son. Police responding to the scene. Seneca county sheriff lieutenant John Cleere. It was right inside this barn here? Yes. Reporter: At first, it didn't appear to be anything out of the ordinary. No. It appeared to be an accident. Reporter: A horrible accident. Yeah, very tragic. Reporter: The parents were upset? Oh, yeah. Absolutely out of their minds. Reporter: But this story would hardly end here. So many twists and turns long after that 911 call. Years after that horrific accident first hit the local paper in Seneca county. The gateway to the sparkling finger Lakes of upstate New York. In fact, down this rambling country road from the Karlsen home is Seneca falls, long thought to be the setting that inspired the 1946 American classic, "It's a wonderful life." Yeah! Hello, Bedford falls! Reporter: But piece by piece, it would soon become clear that this had not been a wonderful life for that young man found in the garage. Four years after what seemed to be an accident, a bombshell call to police, a call on the tip line from a member of Levi's own family, telling that investigator, follow the insurance money. So you were the one who answered the call? Yes. Reporter: And this investigator was soon on his way back to that family farm. Do you remember the call to this day? Oh, yeah. Reporter: The lieutenant telling us the clues were immediate. Red flag number one, they discover a brand new insurance policy for Levi taken out just 17 days before he died. That there's this large insurance policy that was collected on over $700,000 that was taken out only 17 days before the death for the young man in the garage. Reporter: Is that typical that someone that young would have an insurance policy? It seems unusual that a young man in his early 20s would have one that large. Reporter: And then red flag number two. But who was the beneficiary? His father, Karl. Reporter: Why would you name your father the beneficiary when you have your own children? I think we were asking that same question, David. Reporter: And they were soon asking something else. Just listen to red flag number three. Reporter: You discovered a note written by Levi? Yes. Reporter: That Levi had left, saying what? His father was gonna be the sole executor of his estate and controlled, you know, basically dispersed the money to his kids, and that he did not wanna be resuscitated. Reporter: When was the letter notarized? The day of his death. Reporter: The very same morning. And while investigators now believed the evidence was damning, Karl Karlsen planned his own son's death, the whole town tried to wrap their heads around this sinister plot, a plot Levi's sister suspected from the very start. My father is a sociopath. The only one that matters to him is him. We were just pawns in his game. Reporter: Erin's brother's death, she insists, was cold-blooded murder. He didn't die instantaneously. He had time to sit there and understand what was happening to him. Reporter: And that the killer she long suspected inside their own home, their father. For years, Erin says they grew up in fear of their father. And unlike his sisters, Levi stayed in that small town where they were raised. He had two young daughters of his own, but he was still under the thumb of a father who Erin says he was desperate to please. He was basically an indentured servant to my parents. And they were, I don't know who they were, controlling everything. He wanted more than anything to just have a close relationship with our father. He was really striving for that. Reporter: But she is convinced that as her brother tried to build that relationship, her father was crafting something else, the plan to kill his son. You know, to think that as a parent, you can drop a 5,000-pound vehicle on your child and watch him die, it's absolutely unimaginable. Especially as a parent myself, I just can't even begin to imagine. Reporter: We head to the local scrapyard where Levi's pickup truck was taken after that accident. His truck destroyed long ago. No one knows exactly how that pickup was jacked up, but Joshua trout, who knows the family, also knows the weight of one of those trucks. Reporter: I mean, would you go under this pickup truck? I would never go under a truck. I don't like going underneath trucks when they're sitting all four tires on the ground. Reporter: Investigators do believe there was a railroad jack that day holding the pickup truck up, but they're unsure if there was anything else. What they do know, that truck came crashing down. You wouldn't stand a chance. You, you're, you're gone. I mean, you're gone. Reporter: And just when investigators begin asking how a father could do this, the evidence of a pattern, they say, begins to mount, that Karl Karlsen had cashed in on insurance claims before. August 1986, a car fire. The cause, electrical. The insurance payout, $10,000. November 2002, a barn fire that killed three prized horses. The cause, also electrical. The insurance payout, $115,000. But investigators were also learning of the biggest bombshell. What happened to Karl Karlsen's first wife, Levi's mother? After all of the evidence you found right here on this property, what then did you make of what happened to his first wife? Suspicious.
This transcript has been automatically generated and may not be 100% accurate.