Nathan Carman
Nathan Carman with his mother Linda Facebook

A 22-year-old man who rescued at sea earlier this week following an ill-fated fishing trip which left his mother presumed dead, was once a suspect in his millionaire grandfather's fatal shooting. Nathan Carman was found alive on a raft by a Chinese freighter on Sunday (25 September), about 115 miles from Martha's Vineyard after eight days at sea.

Carman's aluminium fishing boat, known as Chicken Pox, sank during a trip with his mother, Linda Carman. After a six-day search for the pair, the US Coast Guard suspended efforts to locate them after trawling 62,000 square miles, according to NBC Connecticut.

Linda Carman is presumed dead. Authorities said her son ventured about 80 miles further out than she had anticipated and are looking into what mechanical repairs Nathan Carman made "upon his own volition which could have potentially rendered the boat unsafe for operation," according to an affidavit.

Carman, who has Asperger's syndrome, was once a suspect in his wealthy grandfather's unsolved death after he was killed in 2013. A police affidavit filed in court revealed that he had bought a rifle consistent with the one used in John Chakalos' slaying.

Chakalos had left an estate worth $42m (£32.3m) to his four adult daughters, including Linda Carman. An arrest warrant was submitted to a prosecutor in 2014, but was returned unsigned to police with a request for further information. Carman was never charged and said he had "absolutely nothing" to do with his grandfather's death.

"My grandfather was the closest person to me. He was like a father to me and I know I was like a son to him," Carman said. "I know that my grandfather was the biggest victim in his homicide but it feels like I was the second biggest victim cause I lost the most important person in my world totally."

In a recent telephone interview, Carman said he is "trying to come to terms with what seems like a fact: that my mum really is gone. My understanding when we went out was that the boat was seaworthy."

"Clearly something happened, but I don't know what, and I don't know how it could have been prevented," he said. Recalling the sinking, he said: "It was just very sudden and very shocking to be in the water, to first figure out where I was, and then everything after that: trying to find my mum, signal for her, and not being able to do that the next day and the next day until I was rescued."

He denied ultimate responsibility for his mother's demise. "I know I wasn't responsible for the boat sinking. I know that I wasn't responsible for anything that resulted from the boat sinking. I know I wasn't responsible for my mum's death," he told ABC News. "But at the same time I feel like I was responsible for my mum and I being out there and in the situation. If I hadn't asked my mum to go fishing with me that weekend, she would still be alive with me today."