The Vermont man charged with killing his mother off the coast of New England in a scheme to inherit millions of dollars has died in jail while awaiting trial, federal authorities said Thursday.

Nathan Carman, 29, pleaded not guilty last year to fraud and first-degree murder in the death of his mother, Linda Carman, and was scheduled to go on trial in October.

An eight-count indictment also says Carman shot and killed his wealthy grandfather John Chakalos as he slept in 2013, in order to obtain money and property from his grandfather’s estate. But the indictment does not charge Carman with his grandfather’s killing, and he had consistently denied any involvement in the two deaths.

The cause of Carman’s death was not immediately clear. He was the sole occupant of a county jail cell in New Hampshire when guards found him dead at around 2.30am, said Doug Losue, superintendent of the Cheshire Corrections Department, which runs the facility. Losue said the death was being investigated by police in Keene, which is near the Vermont state line.

Federal agents execute a search warrant at Nathan Carman's residence. Photo /AP

The US Marshals Service confirmed Carman died Thursday in their custody, adding that the service does not own or operate detention facilities but partners with state and local governments to house approximately 65 per cent of its prisoner population.

One of his lawyers, Martin Minnella, said Carman appeared “in good spirits” when he last spoke to his defence team on the phone Wednesday.

“We were meeting with some experts today over Zoom at 12 o’clock. We were prepared to start picking a jury on Oct. 10 and we were confident we were going to win,” Minnella said. “I’m just heartbroken because I wanted him to have his day in court.”

Carman left a note for his lawyers, but they did not know what it said as of Thursday afternoon, said David Sullivan, Carman’s other attorney. Federal prosecutors told them about the note.

Prosecutors say the deaths of Carman’s mother and grandfather paved the way for him to inherit an estimated $7 million — Linda Carman’s share of her father’s estate. That inheritance remains tied up in probate court in Connecticut, where his three aunts sought to block Carman from receiving any money from his grandfather’s estate.

Chakalos’ three surviving daughters — Carman’s aunts — said in a statement Thursday that they were “deeply saddened” to hear about his death and asked for privacy “while we process this shocking news and its impact on the tragic events surrounding the last several years”.

In September 2016, Carman organised a fishing trip with his mother, who lived in Middletown, Connecticut, during which prosecutors say he planned to kill her and report that his boat sank and his mother disappeared in the accident.

He was found floating in an inflatable raft eight days after leaving a Rhode Island marina with his mother, whose body was never recovered. Prosecutors allege he altered the boat to make it more likely to sink. Carman denied that allegation.

Minnella and Sullivan criticised the indictment — including allegations Carman killed his grandfather, saying he was never charged with that crime.

“The whole situation would have come out in court,” Minnella said Thursday. “This young man would have been vindicated.”

Prosecutors say the inheritance scheme spanned nearly a decade and began with Carman buying a rifle in New Hampshire, which he allegedly used to shoot Chakalos in the man’s home in Windsor, Connecticut on December 20, 2013. Carman then discarded his own computer’s hard drive and the GPS unit in his truck, prosecutors said.

Police say Carman was the last person to see his grandfather alive and owned a semi-automatic rifle similar to the one used to kill Chakalos — but the firearm disappeared. In 2014, police in Windsor drafted an arrest warrant charging Carman with murder in his grandfather’s death, but a state prosecutor declined to sign it and requested more information. No criminal charges were brought until the federal indictment.

In court documents filed in 2018, Carman alleged there was stronger evidence that a woman he described as his grandfather’s “mistress” was involved in the killing, suggesting a robbery motive because of Chakalos’ wealth.

After Chakalos died , Carman received $550,000 from two bank accounts his grandfather had set up and that he was the beneficiary of when Chakalos died. He moved from an apartment in Bloomfield, Connecticut, to Vernon, Vermont, in 2014.

He was unemployed much of the time and by the fall of 2016 was low on funds, prosecutors said, which is when he arranged the fishing trip with his mother.

In 2017, investigators began keeping tabs on a lawsuit filed in federal court in Providence, Rhode Island, where insurers and Carman were suing each other over his rejected $85,000 claim for the loss of his boat, named the Chicken Pox. The insurance case tied all the evidence together and may have spurred a new effort to charge Carman, current and former investigators said.

The insurers’ lawyers laid out a case accusing Carman of plotting both killings and covering them up, using police investigation findings and information they obtained themselves. A judge sided with the insurers in their rejection of Carman’s claim.