BOSTON (AP) — 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 2013 slaying of his rich grandfather, according to court documents that add to the multitude of questions swirling around him and what happened at sea.

Nathan Carman was picked up by a freighter Sunday 100 miles off the Massachusetts coast after what he said was a week adrift that began when his 31-foot 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. Carman has not been charged with anything, and his attorney said his mother's death was a "tragic accident."

Court documents indicate Carman had a history of violence as a child and came under suspicion in the killing nearly three years ago of his maternal grandfather, 87-year-old John Chakalos, a real estate developer who was found shot in his home in Windsor, Connecticut.

A 2014 search warrant obtained by The Associated Press said that Carman had dinner with his grandfather the night before and was the last person known to have seen him 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 in the slaying. 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 $42 million to his four adult daughters, including Carman's mother.

Windsor police Capt. Thomas LePore said Wednesday that the case is still open and that Carman remains a "person of interest."

In the course of investigating the killing, authorities said in court papers that they found Carman's handwritten notes on making explosives, seized a shotgun and other weaponry from his Middletown, Connecticut, apartment, and learned from family members that he once held another child "hostage" with a knife.

They also said Carman had had several alarming episodes while he was a high school student in Connecticut. Those episodes were not explained.

Authorities would not discuss the investigation into the boating trip.

Mother and son set off from a marina in South Kingstown, Rhode Island, on Sept. 17, authorities said. Carman told the Coast Guard that their boat sank the next day after he heard a "funny noise" in the engine compartment and saw water pouring in.

He said he lost sight of his mother, 54-year-old Linda Carman, before he managed to make it into the four-person life raft with food and water.

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 off-shore 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.

Outside his Vermont home Tuesday, Carman said he had been through "a huge amount" emotionally, and he thanked the public for its concern and prayers. His attorney, Hubert Santos, said Carman "fully cooperated" with the Coast Guard.

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Associated Press writers Michael Melia in Hartford, Connecticut; and Jennifer McDermott in South Kingstown, Rhode Island, contributed to this report.