PROVIDENCE, R.I. (WPRI) — A Vermont man has been charged with killing his mother during a 2016 fishing trip off the coast of Rhode Island.

Nathan Carman, 28, of Vernon, was arrested Tuesday on an indictment charging him with murder on the high seas and several counts of fraud, the U.S. Attorney’s Office for the district of Vermont announced.

He’s scheduled to be arraigned in federal court Wednesday.

The unsealed grand jury indictment claims Carman killed his mother Linda, as well as his millionaire grandfather John Chakalos three years prior, in an effort to inherit his family’s estate. He has previously denied the allegations.

While the indictment claims he killed both relatives, he is only charged with murdering his mother. He faces fraud charges related to the Chakalos’ death.

“Nathan Carman devised a scheme to defraud the Estate of John Chakalos, its executor, the Dynasty Trust, and its trustees, and to obtain money from the Dynasty Trust by materially false and fraudulent pretenses, representations, and promises,” the indictment reads. “As a central part of this scheme, Nathan Carman murdered John Chakalos and Linda Carman.”

“He concocted cover stories to conceal his involvement in those killings. As part of his cover-up, Nathan Carman misrepresented his involvement in and responsibility for those deaths to law enforcement, to his family, to others who made inquiries about the deaths and their circumstances, and to others who challenged his cover-up or challenged his rights to his grandfather’s assets,” the charging document continues.

Carman received more than half a million dollars as a result of his grandfather’s death, according to the indictment, which claims he shot Chakalos twice with a rifle he purchased in New Hampshire.

In 2016, Carman and his mother left for the trip on his boat, the Chicken Pox, off the coast of South Kingstown. When they didn’t return the next day as planned, a massive search-and-rescue effort was conducted by the Coast Guard and other agencies.

Nathan and Linda Carman

Carman, who was 22 at the time, was found alone on a life raft about 100 nautical miles south of Martha’s Vineyard about a week after the boat sank.

Linda Carman, 54, was never found. Prosecutors claim Carman intentionally sank the boat and evaded detection by the search teams while floating on the life raft.

Nathan Carman was considered a suspect but never charged in his grandfather’s murder. He suggested in court filings that Chakalos may have been killed by an unnamed mistress.

Nathan Carman was later sued over an $85,000 insurance claim he made on the boat before it went down. A judge ruled against him, saying that “faulty repairs” Carman made to the vessel contributed to its sinking.

Court documents show Carman is charged with one count of first-degree murder, three counts of frauds and swindles, and four counts of fraud by wire, radio, or television, all of which are felonies.

If he’s convicted of murder, he faces a mandatory life sentence, according to the U.S. Attorney’s Office, while the fraud charges each carry a potential penalty of up to 30 years in prison.