Tonight, it's the explosive case about to go to trial, a son crushed under a 5,000 pound pickup truck, the dramatic call to 911, the parents and their cries for help. Was the accident really a... See More
Tonight, it's the explosive case about to go to trial, a son crushed under a 5,000 pound pickup truck, the dramatic call to 911, the parents and their cries for help. Was the accident really a surprise to everyone? Because when investigators start digging a shocking discovery. Was there a deadly pattern to this family after all? Tonight here the 20/20 exclusive, the daughter stepping forward to say, I know what you did. 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. So he was very mechanical. Reporter: And on one of those crisp fall days back in november 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 step-son. The truck fell on your step-son? 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 want to start cpr. Do you know cpr? 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: And, at first it didn't appear to be anything out of the ordinary. No, it appeared to be an accident. Reporter: A horrible accident. And 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. And long after 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." 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 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 told us the clues were immediate. Red flag number one -- they discovered a brand new insurance policy for levi taken out 17 days before he died. Well, that there's this large insurance policy that was collected on over $700,000 that was taken out only 17 days before his death. Reporter: For the young man in the garage. 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: Then, red flag number two. And who was the beneficiary? His father, mr. Karlsen. 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: They soon began asking something else. Just listen to red flag number three. You discovered a note written by levi? Yes. Reporter: That levi had left, saying what? That his father was going to be the sole executer of this estate and controlled, you know, basically dispersed the money to his kids, and that he did not want to be resuscitated. Reporter: When was the letter notarized? The day of his death. Reporter: The very same morning. And while investigators believe the evidence is damning, that karl karlsen planned his son's death, the whole town now trying to wrap their heads around this sinister plot. A plot levi's sister suspected from the very start. And tonight, for the first time, she's coming forward in a "20/20" exclusive. My father is a sociopath. The only one that matters to him is him. We were pawns in his game. Reporter: Her brother's death of cold blooded murder. He had time to sit there and understand what was happening to him. Reporter: And the killer she long suspected inside their own home, her father. She says they grew up in fear of their father and levi stayed in that town where they were raised, he had two young daughters, but 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 I don't know who they were, controlling everything. He wanted more than anything to just have a a close relationship with our father. He was really striving for that. Reporter: She is convinced her father was crafting something else. The plan to kill his son. 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 scrap yard, where levi's pickup truck was taken to 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. 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. Investigators do believe there was a railroad jack that day. Unsure if there was anything else holding up that truck. What they do know is it came crashing down. You wouldn't stand a chance. You're gone. I mean, you're gone. Reporter: When investigators begin asking how a father could do this, evidence of a pattern begins to mount. Karl karl sen cashed in on insurance claims before. August 1986, a car fire, the cause electrical, insurance payout $10,000. November 2002. A barn fire that killed three prized horses. Cause, also electrical. 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 there on the 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.