Nathan Carman, saved after he said he spent a week adrift when boat sank, said he had nothing to do with grandfather’s death and did all he could to find mother

A 22-year-old man rescued from a life raft after a fishing trip that left his mother missing and presumed dead had been a suspect in the still unsolved 2013 killing of his rich grandfather, adding to the multitude of questions swirling around him and what happened at sea.

Nathan Carman was picked up by a freighter on Sunday, 100 miles off the Massachusetts coast, after what he said was a week adrift that began when his 31ft aluminum fishing boat inexplicably sank during a mother-and-son outing.

Coast guard officials interviewed Carman, and police searched his home in Vermont as part of an investigation into the ill-fated trip. He has not been charged with anything.

In an interview with the Associated Press on Wednesday, he said he had “absolutely nothing” to do with his grandfather’s killing and did everything he could to find his mother, 54-year-old Linda Carman, of Middletown, Connecticut, as their boat went down. He said he blew a whistle and called out frantically for her for hours.

“I was yelling, ‘Mom! Mom!’” Carman said. He added: “I loved my mother and my mother loved me.”

According to court documents, Carman came under suspicion in the killing three years ago of his maternal grandfather, 87-year-old John Chakalos, a wealthy real estate developer who was found shot dead in his Windsor, Connecticut, home.

A 2014 search warrant obtained by the AP said that Carman was the last person known to have seen Chakalos alive; that Carman had bought a rifle consistent with the one used in the crime; and that he discarded his hard drive and GPS unit used around the time of the shooting.

Carman was never charged. According to court papers, police submitted an arrest warrant to a prosecutor, but it was returned unsigned with a request for more information.

In his will, Chakalos left an estate worth more than $42m to his four adult daughters, including Carman’s mother.

Capt Thomas LePore of Windsor police said on Wednesday that the case was still open and that Carman remained a “person of interest”.

“My grandfather was like a father to me, and I was like a son to him,” Carman told the AP. “He was the closest person in the world to me, and I loved him and he loved me, and I had absolutely nothing to do with his death.”

In the course of investigating the killing, authorities said in court papers that they learned from family members that Carman had a history of violence as a child, including one incident in which he allegedly held another child “hostage” with a knife. The documents also said Carman had several alarming episodes while he was a high school student, although no details of those incidents were given.

Authorities would not discuss the investigation into the boating trip.

Mother and son set off from a marina in South Kingstown, Rhode Island, on 17 September, authorities said. Carman told the AP that their boat sank in a matter of minutes around noon the next day after he heard a “funny noise” in the engine compartment and saw water pouring in.

He said he saw his mother in the cockpit and then saw her pulling in the fishing lines. He said he grabbed three bags containing food, flares and life jackets. But when he looked back, his mother was no longer there, he said.

“One minute I was standing on the deck, the next minute I was in the water,” he said.

Carman said the life raft self-inflated, and he swam to it, about 15-20ft away, and began calling for his mother.

On Monday, authorities searched Carman’s home in Vernon, Vermont, and seized a modem, a SIM card and a letter. Their search warrant indicated investigators think that Carman was handling some boat motor repairs himself and that the vessel might not have been seaworthy.

“The investigation has also revealed that Nathan had intended to go fishing further offshore in a different location than what were his mother’s intentions and understanding,” the warrant said.

Family members have said Carman has Asperger’s syndrome, a form of autism that can be characterized by social awkwardness and repetitive behavior. Experts say people with Asperger’s are no more likely than others to commit violent crimes.

His attorney, Hubert Santos, said that Carman cooperated fully with the coast guard and that his mother’s death was a “tragic accident”.