A Vermont man lured his mother on a fishing trip off the coast of Rhode Island in 2016, killed her and sank the boat in a scheme to inherit his family’s estate, federal prosecutors said on Tuesday.

An indictment unsealed in federal court in Burlington, Vt., on Tuesday, accuses Nathan Carman, 28, of Vernon, Vt., of murdering his mother, Linda Carman, while boating in September 2016 and making false reports to the authorities about what had happened on the high seas.

Federal prosecutors do not specify how they believe Mr. Carman killed his mother, but they say his boat, named Chicken Pox, was purposefully sunk. Mr. Carman spent eight days adrift at sea before the crew of a commercial ship, the Orient Lucky, found him floating on a life raft.

In 2013, as another part of his scheme, Mr. Carman grabbed his Sig Sauer rifle and shot and killed his grandfather, John Chakalos, who became wealthy by building and renting nursing homes and other real estate ventures, in Windsor, Conn., the indictment states. Mr. Carman was not charged with that killing, according to the indictment.

“As a central part of this scheme, Nathan Carman murdered John Chakalos and Linda Carman,” the indictment states. “He concocted cover stories to conceal his involvement in those killings.”

The indictment does not state how much money Mr. Carman could have inherited. The Associated Press reported that as his mother’s only heir, he would have received $7 million.

One of the eight criminal counts that Mr. Carman faces accuses him of murdering his mother, who had “a strained relationship” with her son, said the U.S. Attorney’s Office for the District of Vermont. The seven others are related to Mr. Carman’s attempts to get money from Mr. Chakalos’s estate, which was worth tens of millions of dollars, or from insurance companies.

Mr. Carman was arrested on Tuesday and could not be reached for comment. The Office of the Federal Public Defender for the District of Vermont, which is representing Mr. Carman, did not respond to a phone call seeking comment on Tuesday. He is scheduled to be arraigned on Wednesday.

William Michael, a lawyer for Ms. Carman’s three sisters, who sued Mr. Carman in 2018 to block him from inheriting money and accused him of killing their father and sister, did not immediately respond to a phone call seeking comment on Tuesday. Their lawsuit is still pending, according to The Associated Press.

Suspicions about Ms. Carman’s death first arose in September 2016 when the Coast Guard found Mr. Carman on an inflatable life raft without his mother. Her body was never recovered.

They had set out on a fishing trip after 11 p.m. on Sept. 17, 2016, to spend time together on Mr. Carman’s boat. It was Ms. Carman’s “principal way of interacting with her son,” the indictment states.

By that time, Mr. Chakalos had been dead for three years, and Mr. Carman had already received $550,000 as a result. The money had been mostly spent by fall 2016 because he had been unemployed for long stretches, the indictment states.

Federal prosecutors say that Mr. Carman had planned and prepared for the killing of his mother in several ways: He removed trim tabs — metal plates that help to stabilize performance — from the boat and removed his computer from his home to prevent officials from reviewing it while he was away.

He also planned to report the sinking of the Chicken Pox and Ms. Carman’s disappearance as “accidents,” the indictment states.

Ms. Carman had “believed that she would be returning home by noon the next day, as evidenced by the float plan she left with friends,” the indictment states.

Mr. Carman told The Associated Press in 2016 that “what happened on the boat was a terrible tragedy that I am still trying to process and that I am still trying to come to terms with.”

He added that he did not know “what to make of people being suspicious,” and that he had “enough to deal with.”

The indictment also states that Mr. Carman tried to defraud the company that insured his fishing boat.

The U.S. attorney’s office did not say why their investigation lasted years and did not immediately respond to an email seeking comment on Tuesday.

In 2019, a federal judge in Rhode Island ruled that Mr. Carman contributed to the Chicken Pox sinking, according to The Providence Journal.

If convicted of the murder charge, Mr. Carman would face life in prison.