Police are looking into the mysterious circumstances surrounding a U.S. man found drifting off the coast of Massachusetts last month, after an apparently disastrous mother-son fishing trip.

Nathan Carman, 22, was plucked from the waters of the Atlantic by a passing freighter called the "Orient Lucky" on Sept. 25, approximately 185 kilometres south of Martha's Vineyard, Mass. Carman told the cargo ship's crew that he had been out fishing with his mother when something went awry with the boat.

Nathan and Linda Carman, 54, originally set out from Rhode Island on their 9.5-metre-long boat on Sept. 17, according to officials.

"They developed some kind of problem with the boat, (and) the boat sunk," Tom O'Reilly, a port agent in Saint John, N.B., told CTV Atlantic.

"I feel healthy," Carman told reporters after his rescue. "Emotionally, I've been through a huge amount."

But the case has aroused suspicion in the northeastern U.S., where local, state and federal officials are working together to investigate Linda Carman's disappearance.

Investigators searched Nathan Carman's home in Vermont the day after he was recovered, and confiscated a modem, a cellphone SIM card and a letter. Police suspected Carman had attempted to repair the boat motor, and that he may have done a poor job of it, according to a search warrant obtained by The Associated Press.

The case has also dredged up the unsolved killing of Nathan Carman's grandfather and Linda Carman's father, John Chakalos, who was found shot to death in his home in Windsor, Conn., in 2013. Chakalos was a wealthy real estate developer.

Police had applied for an arrest warrant charging Nathan Carman with the murder of his grandfather, but a prosecutor did not sign the warrant and requested more information, according to court documents reported on by The Associated Press.

Nathan Carman has denied any involvement with the 2013 killing.

With files from CTV Atlantic and The Associated Press