The family of a 23-year-old man who was found after seven days missing at sea is trying to prevent him from inheriting millions following his wealthy grandfather’s homicide.

Attorneys for the aunts of Nathan Carman filed a petition Monday that alleges he killed his grandfather “out of malice and greed” and asks the court to block the sizeable inheritance he now stands to gain.

The petition, which lists the three sisters of Nathan’s mother, Linda Carman, as petitioners, is known as a “slayer action,” which seeks to prohibit a person from receiving an inheritance from someone they killed. The filing points the finger at Nathan for his grandfather’s killing, asking the court to “declare that the murderer was Nathan Carman.”

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John Chakalos, an 87-year-old real estate mogul who owned a number of nursing homes across New England, was found shot to death in his Windsor, Connecticut, home in 2013. Nathan is the last known person to see his grandfather alive.

Nathan was also the last person to see his mother before she disappeared at sea last year. Nathan and Linda Carman were first reported missing Sept. 18, 2016, after they failed to return from a fishing trip off the Rhode Island coast in his boat named “Chicken Pox.”

Nathan was rescued a week later from a life raft more than 100 nautical miles from Martha’s Vineyard. Linda has never been found. Nathan claims the boat sank after rapidly taking on water and that he never saw his mother as he entered the life raft, which was enclosed and stocked with food and water.

The lawsuit is the first time the Chakalos sisters have publicly accused their nephew in the killing. Nathan has never been charged in either his grandfather’s homicide or his mother’s disappearance, though police have called him the prime suspect in the former case.

In an exclusive interview with ABC News’ Linzie Janis, Nathan denied any involvement in his grandfather’s death, saying “he was like a father to me.” Nathan also claimed he did nothing to intentionally sink the boat with his mother on board.

However, an investigation by an insurance company blamed Nathan for the incident, claiming he took “intentional” actions that caused the boat to sink.

The lawsuit asks for Nathan’s inheritance to be placed into a trust that would be controlled by his aunts. However, the action is about justice rather than money, according to their attorney. “The surviving sisters cannot stand idle while their father's killer, and perhaps their sister's killer also, profits from his actions,” said Dan Small in a statement.

Should they win the case, Small says the sisters plan to use the money to pay for expenses related to the investigations into the death of their father and disappearance of their sister, with the rest going to charity.

Attorneys for Nathan Carman did not respond to a request for comment.

ABC's Linzie Janis and Enjoli Francis contributed to this report.