Rescued boater's family claims he killed grandfather

The family of Nathan Carman, 23, who was found after seven days missing at sea, is trying to prevent him from inheriting millions following his wealthy grandfather's homicide.
4:56 | 07/18/17

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Transcript for Rescued boater's family claims he killed grandfather
We are back now with that new twist in the story making national headlines involving Nathan Carman, a young man rescued at sea after his boat sank. His mother still missing. Officials calling that suspicious. Now Nathan is facing new accusations in a legal battle over a multimillion dollar estate. ABC's Linzie Janis has been covering this from the very beginning. Good morning, Linzie. Reporter: Nathan Carman has not been charged with any crime but his aunts say they believe he is responsible for his mother's death and say there is evidence he murdered his grandfather years earlier. They're asking a civil court judge to declare him a murderer so he can't collect his inheritance. This morning, the family of Nathan Carman who was rescued after being lost at sea for more than a week is trying to stop him from inheriting millions of dollars from his missing mother and murdered grandfather. Attorneys for Carman's three aunts filing a petition alleging their nephew was the last to see both family members alive. Asking a civil court judge to invoke the slayer rule, prohibiting a person from inheriting money from someone they killed. Family is alleging that the evidence indicates that Nathan Carman killed John chakalos and he should not profit from his actions. Reporter: Nathan has not been charged with any crime and denies any involvement in the 2013 killing of his wealthy grandfather, John chakalos. I had absolutely no involvement in my grandfather's murder. Reporter: At the time investigators considering Carman their chief suspect. A search warrant pointing to a gun they say he purchased but was never recovered. The same type as the murder weapon. As for his mother Linda, Nathan says they were nearly 100 miles offshore last September when their boat began taking on water. Telling us, it sank so quickly, he didn't have time to make a mayday CL. I got on board the life raft and was looking around and I was calling out to my mom. I did not see or hear my mom. Reporter: Within hours of his dramatic rescue by a Chinese freighter police once again not buying his story. Searching his Vermont home for evidence. He deliberately sank the boat. Multiple law enforcement agencies are still investigating the disappearance of Linda Carman and the murder of John chakalos. We reached out to flay than's lawyer but have not heard back. Last year his lawyer told me Nathan is innocent and Nathan is expected to inherit $7 million to $8 million. I remember that hour that you did for "20/20," absolutely riveting about this. We bring in ABC news chief legal analyst Dan Abrams. An insurance company denied him benefits. What's the case here. In the insurance case what they said he made changes to the boat intentionally, right before they went out to sea, that made the boat less safe. They said that those intentional acts by him meant that they shouldn't have to pay any insurance benefits. But they didn't say he killed his mother or he killed his grandfather. Here you have the family members going a step further and saying we believe he killed his mother, he killed his grandfather and as a result shouldn't be able to get any of those inheritance dollars that he would ordinarily be entitled to. Tell us how a civil court works. Unlike in a criminal case where you have to prove something beyond a reasonable doubt. Here they have to prove it's more likely than not and there are these statutes in place in a lot of states. Actually don't have one in new Hampshire but in most states basically says if you're the one who took someone's life, you can't then benefit when it comes to an estate. And so what they're saying here is basically the case law in new Hampshire makes it clear that in a case like this, if they can prove that it's more likely than not that he killed his mother, killed his grandfather, he shouldn't be able to get the benefit of the grandfather's estate. As I said I remember watching Linzie's piece and she spent a lot of time with Nathan and really got into the whole re-create, this, that and the other and the family feels there's such compelling evidence. You would think if there is all this evidence, he hasn't been charged with anything. It's a great question and the answer is this goes back to the difference in a legal standard. In a criminal case the government has to be convinced it can prove beyond a reasonable doubt that he committed a murder. In this case all they have to prove is that it's more likely than not. If he is found responsible in this case, he doesn't go to jail, he doesn't go to prison, he just loses his inheritance. So the stakes are very, very different and it's a super interesting legal case, as well. All right. Thank you, Dan.

This transcript has been automatically generated and may not be 100% accurate.

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