By Audrey Karnan

Newly obtained investigative files provide fresh insight into one of New England’s most notorious murder mysteries, which unfolded after Nathan Carman and his mother Linda Carman took a fateful fishing trip on Sept. 17, 2016.

Nathan said the boat took on water and suddenly sank. The 22-year-old survived and was rescued at sea, but Linda was nowhere to be found and presumed dead.

As officials investigated Nathan’s story, they began to doubt his account of the events that unfolded out at sea. They suspected he may have purposefully sunk the boat and killed his mother, whose body was never recovered. Nathan denied the allegations, insisting the sinking was an accident.

A new “20/20” episode, “Family Lies?”, airing Friday, April 4, at 9 p.m. ET on ABC and streaming the next day on Hulu, examines the case.

You can also get more behind-the-scenes of each week’s episode by listening to “20/20: The After Show” weekly series right on your 20/20 podcast feed on Mondays, hosted by “20/20” co-anchor Deborah Roberts.

Now, in a new “20/20” investigation, detectives reveal how they worked on a case that spanned the Northeast and shed light on a family they believed was fractured by wealth.

“We had no boat, we had no body,” retired FBI Special Agent Lisa Tutty told “20/20.” “But as in any investigation, there’s always evidence that can be recovered. It’s just a matter of making sure that you know where to look.”

Nathan Carmen spoke in 2017 for an ABC News “20/20” interview.

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Nathan grew up in Connecticut, the son of Linda and Clark Carman, who divorced when he was young. His adolescence was marked by loneliness and difficulty socializing with others. Nathan was diagnosed with Asperger’s — part of a broader diagnosis of Autism Spectrum Disorder, which can lead to difficulty with social interactions and communication skills.

His grandfather was John Chakalos, a wealthy real estate developer who made his fortune building nursing homes and care facilities across New England. Chakalos had four daughters, but Nathan had a special place in his heart as the first male grandson. The 87-year-old widower doted on him, paying for his schooling, housing, and even a horse.

On Dec. 20, 2013, Chakalos was found fatally shot in the head inside his Windsor, Connecticut, home.

“20/20” spoke with Christopher McKee, a former lieutenant with the Windsor Police Department who was dispatched to the scene that morning in 2013.

“There had been large sums of money in the house and other valuables, and honestly, nothing appeared to be touched,” McKee said. “It told me that this was personal.”

According to ballistics tests conducted on the bullet fragments found at the crime scene, the killer most likely used a Sig Sauer rifle to commit the murder.

When police initially asked Nathan if he owned any firearms, he said he only had an air gun, but authorities later determined that he purchased a Sig Sauer in New Hampshire weeks before the murder.

“Well, he acknowledged that he bought this weapon.” McKee told “20/20.” “When asked about it, he said he lost it, and he didn’t know where the gun was.”

Nathan told police he bought the gun because he was not feeling safe at home and denied having anything to do with his grandfather’s murder.

The gun has never been found.

Investigators also looked into Nathan’s alibi for that night. McKee told “20/20” that there was at least one hour during the timeframe when Chakalos was shot that Nathan couldn’t account for his whereabouts.

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Nathan said he left his house around 3 a.m. to meet his mom for one of their fishing trips, but on the way, he claimed he got lost. He wasn’t heard from until 4:01 a.m.

The police never found the murder weapon. Without this key evidence, they do not have a strong enough case to make an arrest. Nathan was never charged with his grandfather’s murder.

In an exclusive interview with “20/20,” Charlene Gallagher, Chakalos’ daughter and Nathan’s aunt, said that she would like people to remember her dad for his kindness and love.

“He’d give you a hard time,” she remembered. “But he always did it out of love.”

After Chakalos’ murder, Nathan’s relationship with his extended family was fraught. He used some of his inheritance from his grandfather’s death to purchase a house in Vermont and a fishing boat, “The Chicken Pox.”

Casey Sherman, who wrote the recent book “Blood in the Water” about the case, said Linda frequently fished with Nathan to nurture her relationship with her son, even though she did not eat fish and was nervous about going out on the water.

Chuck LaPenna, Linda’s cousin, still remembered his reaction when he found out that they were missing.

“I told her not to go on that boat,” he told “20/20.”

David Farrell, who represented the boat insurance companies in matters related to Nathan Carman, speaks with ABC News’ Deborah Roberts.

While the Coast Guard searched 64,000 square nautical miles for Nathan and Linda, investigators back on land were surprised by what they found in Nathan’s car. He had a bucket of eels, a common bait used when fishing for striped bass.

“If you left the dock and didn’t take your bait, then what are you using for bait if you’re going fishing?” U.S. Coast Guard Special Agent Eric Gempp told “20/20.”

According to the FBI, Nathan had also recently purchased an anchor that would never have worked for his boat, along with some lengths of chain.

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Alfred Bucco III, deputy chief of the South Kingstown Police Department, interviewed Nathan shortly after being rescued. It was the last time Nathan spoke with law enforcement about what he says happened on the boat.

“I knew there was water in the bilge. I knew there was a lot of water because it was already up to that level, which is why I wanted to bring the safety stuff as a precaution. But I thought I was going to find the problem and fix it,” Nathan told Bucco in the audio recording of their conversation, broadcast for the first time by “20/20.”

Nathan said that he told his mother to bring in the fishing lines. Then, as he was carrying emergency gear out of the wheelhouse, the boat gave way beneath him and suddenly he was in the water, he said. He told Bucco that he did not hear or see Linda.

Charlene Gallagher told “20/20” she would like people to remember her dad, John Chakalos, for his kindness and love.

While police continued to investigate, Nathan filed an $85,000 insurance claim for his lost boat.

“It happened very promptly, and that indicated to me that this guy is really serious,” attorney David Farrell, who represented the boat insurance companies, said.

They denied his claim, citing “holes in his story.”

Nathan said that he did not radio for help or set off the boat’s emergency locator beacon because he did not realize the boat was sinking until it was too late.

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Oceanographer Richard Limeburner analyzed the currents in the area and determined that, based on the data, Nathan’s life raft would have drifted west.

“Nathan claimed he drifted eastward,” Limeburner, who testified on behalf of the insurance companies, said in a “20/20” interview. “That did not make any sense.”

Ultimately, a judge found that Nathan made faulty repairs to the boat and ruled in favor of the insurance companies. However, the ruling did not determine if he intentionally sank the boat or killed his mother.

On May 10, 2022, Nathan was criminally indicted in the murder of his mother, Linda.

“There were too many inconsistencies. There were too many things that just didn’t add up,” Special Agent Gempp said.

Nathan pleaded not guilty.

Before his case went to trial, the Carman family would endure yet another tragedy.

On June 15, 2023, Nathan took his own life in jail. The criminal charges against him were subsequently dismissed.

Chakalos’ murder remains unsolved, but the Windsor Police Department told “20/20” that the case is still active. As for the family inheritance, both Chakalos’ and Linda’s estates are still in probate court.

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“I think he is both a victim and a villain in his own story,” Sherman — the “Blood in the Water” author — said. “Nathan Carman remains an enigma.”

Carman’s aunts chose to attend his funeral, even though they believed he murdered both their father and their sister.

“Nathan should be remembered as a troubled soul,” Gallagher told “20/20.” “I’m relieved that Nathan was buried in the family plot next to my dad. He needs to rest.”

If you are struggling with thoughts of suicide or worried about a friend or loved one, call or text the Suicide & Crisis Lifeline at 988 for free, confidential emotional support 24 hours a day, seven days a week.