Keene, N.H. (AP) — The family of a man suspected in the slaying of his millionaire grandfather filed a lawsuit in New Hampshire on Monday accusing him of killing his grandfather and possibly his mother, who was lost at sea during a mother-son fishing trip, in a bid to collect a multimillion-dollar inheritance.

Nathan Carman’s maternal grandfather, wealthy real estate developer John Chakalos, was found fatally shot at his Connecticut home in 2013. Carman, of Vernon, Vermont, was a suspect in the death of Chakalos, who was 87, but no one was arrested.

Carman, who’s in his early 20s, survived the fishing trip near Rhode Island after the boat carrying him and his mother sank in 2016. His mother, Linda Carman, is presumed dead.

Linda Carman’s three sisters sued in New Hampshire’s Cheshire County, where Chakalos owned another home. They’ve asked a judge to block Nathan Carman from collecting money from his grandfather’s estate. Chakalos left more than $42 million to his four daughters, including Linda Carman.

Nathan Carman has denied any involvement in his grandfather’s death and has said he didn’t sabotage the boat used for the mother-son fishing trip. A cellphone number for him rang unanswered on Monday, and messages left for his attorneys weren’t immediately returned.

The sisters’ attorney, Dan Small, said the details and evidence in Chakalos’ death and in Linda Carman’s disappearance “all point to Nathan as the prime suspect.”

“Yet he now stands to inherit millions of dollars from their estates,” Small said in a statement.

Small said if the sisters win the lawsuit and any of the money that would have gone to Nathan Carman goes to them, “they pledge to use those funds exclusively to pay for expenses incurred relating to the investigation into the death of their father and disappearance of their sister, and any remaining funds will go to charity.”

The lawsuit says that, according to a police investigation, Nathan Carman bought a semi-automatic rifle from a gun store in New Hampshire before his grandfather’s shooting death and it was the same caliber weapon used in the killing. The document says he concealed the information from investigators and now says the weapon is missing.

In February, Nathan Carman told ABC’s “20/20” that he’s misunderstood and an easy target for police because he suffers from Asperger’s syndrome, a condition on the autism spectrum. A few of its hallmarks are awkward social and communication skills.

He said he patched some holes on the 31-foot-long boat with marine putty before going fishing with his mother but insisted the boat was seaworthy. Insurance companies claimed in court last month that “incomplete, improper, and faulty repairs” were made the day before the boat sank.

In the 2013 case, a search warrant said that Nathan Carman was the last person known to have seen Chakalos alive and that he discarded his computer hard drive and a GPS unit used around the time of the shooting.

Carman said last year that his grandfather “was the closest person in the world to me, and I loved him and he loved me.”

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