Transcript for Rescued boater Nathan Carman says he wasn't responsible for mom's death
You're about to meet a young man dubbed the real-life cast away, rescued at sea after his mother disappeared in what seemed to an freak boating accident. He says he expected sympathy. What he got was a police investigation. ABC's Linzie Janis asked some tough questions. Reporter: It looked like an inspiring tale of survival. A young man missing for seven days, found drifting on a life raft. The son rescued off Martha's vineyard -- Reporter: The coast guard combing hundreds of miles of ocean in vain, looking for Nathan Carmen and his mother, Linda. And they just had given up hope when the call came in. Nathan was alive. Found alive in good condition -- Reporter: But at he's pulled aboard the Chinese freighter that spotted him -- When I saw his life raft, I did not see my mom. Have you found her? No, we haven't been able to find her yet. Reporter: There's no sign of Nathan's mother. Just his version of what happened. He arrives on shore under a cloud of suspicion. For some, his story just doesn't add up. I did not cause my mother's death. I loved my mother. Present tense, I love my mother. Reporter: Nathan decides he wants to tell his story and he grants us an exclusive interview. He seems anxious, traumatized, worried he won't look or sound good on camera. There's a reason for this. Nathan has been diagnosed with asperger's syndrome, which is on the autism spectrum. It's associated with social awkwardness and flat, measured speech patterns. Let's begin if we could with the fishing trip. As we talk, it's clear. The conversation will be a fragmented account with plenty of blank spaces. I'm not going to go into that now. I'm not going to answer that. I'm not going to go there. Reporter: But Nathan is able to provide more details about that ordeal at sea. My mom and I fished very frequently together. That's the primary thing that we did to spend time with one another. Reporter: Nathan is from a wealthy new England family. His mother Linda a nurse who worked with autism patients, loved fishing with her son. He says that on September 17th of last year, just before midnight, he and his mom set out on that fateful trip aboard his 31-foot fishing boat named "The chicken pox." I have experience boating. I have experience fishing. I did not have experience offshore fishing. Reporter: Even so, the two of them go 75 miles farther than usual, to catch tuna, to an area called block canyon. It takes them all night to get there. Nathan says the new day was perfect. They had life vests on board but weren't wearing them. Hours passed. It was midday when he says the trouble started. I heard a noise. The belt on the engine was picking up water, kind of spinning it. What did you think? I knew that there was a serious problem. But I didn't think we were sinking. I thought that I was going to diagnose the problem and that we were going to go back to shore. Reporter: He says he told his mother to gather the fishing lines in the rear, but he says, to his shock, in no time the boat was under water. I was walking on the deck, it was there, then it wasn't. Any sign of your mother at this point? No, not at that point. Reporter: He claims he was totally disoriented. I got on board the life raft and was looking around. I was calling out to my mom. I did not see or hear my mom. Reporter: But police aren't buying Nathan's story. Before he even stepped foot on land, they had executed this search warrant on his home in rural Vermont. Looking for evidence of reckless endangerment. Captain David Mccormack runs Irish jig charters out of ram point marina in Rhode Island where Nathan and Linda set sail from. He is puzzled by Nathan's actions after he realized the boat was taking on water. There was a functioning alert system on board. Nathan never used it. How long does it take to activate it? Just a matter of seconds. Just flip the switch, it's activated. It's a manual type. It had a radio. And there was also an emergency position indicating radio beacon. I didn't know that we were sinking. I knew that we had a problem, but I didn't know that we were sinking. Until we sank. Reporter: Investigators see a possible financial incentive for foul play. Is Nathan due to get all of his mother's money? I think it's around $7 million, $8 million. Reporter: Eight days after our first interview, Nathan invites us back to Vermont to set the record straight. Hi, Nathan, how are you? I was lost at sea, my mom died. It would be great to have people embracing you saying, we're glad you're home, we're glad you're alive, and also helping me to deal with my mom's death. It hasn't been that. Reporter: He says he was counting on support from Linda's three sisters, his aunts. Instead, there's been a stony silence. I haven't received any calls. Not from a single one of them? That's correct. It makes me feel like I have no family. Reporter: Thath than's aunts didn't want to speak with us either. Their focus is not on relationships, it's getting answers to questions. Reporter: Nathan grew up 100 miles south of here in middletown, Connecticut, a suburb of Hartford. An only child, his parents divorced when he was very young. His mother, Linda, was everything to him. She was a good person. A warm person. We did have a challenging relationship at one point in my life. But she was the only family who I really had. Reporter: Another source of comfort, Nathan's grandfather, Linda's dad, jock shockless, a multi-millionaire real estate and nursing home developer. He was like a father to me. And I know I was like a son to him. Reporter: But on December 20th, 2013, the 87-year-old was found shot to death in his modest Connecticut home. One of his daughters had gone to check on him. And found him in his bed with what looked to be gunshot wounds to his head and back. Reporter: Cops focus on the last person to see shockless alive, his beloved grandson. The two had dinner together earlier that evening but police say Nathan's whereabouts later that night are unaccounted for. He was definitely our prime suspect. Reporter: And according to this 2014 search warrant, Nathan discarded both the hard drive of his computer and the gps unit used on the morning of December 20th, 2013. That he'd recently bought a 308 caliber rifle, the same caliber used in the homicide of John shockless. Neither Nathan's rifle nor the murder weapon were ever found. Nathan categorically denies having anything to do with the murder. If they asked me, can we look at your hard drive, can we have your gps? At that time when I had -- when they were in my apartment, my answer would have been, sure, gladly, you can take it. But they didn't. Reporter: He says there is no link between his grandfather's death and his mother's disappearance. There's no relationship between my having been the last person, other than the killer, to have seen my grandfather alive, and my having been on the boat with my mother when it sank. Reporter: When we asked Nathan again about what caused the boat to sink, he simply says he doesn't know. I'm not a diesel engine mechanic. Reporter: But boat owner Mike iozi told police he saw Nathan on the dock just hours before his fishing trip. It kind of caught my eye when I saw him leaning over the back and drilling two holes in the transom of the boat. Reporter: He says Nathan told him he was removing stabilization devices, like these. Something called trim taps. Nathan admits removing the trim taps but claims he patched the holes properly with marine putty. Another thing that's happened since we last got together, police searched your mom's home. Yeah, what -- Do you know anything about that? We're done for this evening, period, we're done here. We're not trying to make you uncomfortable, we're trying to give you an opportunity to answer some of these allegations. We're done here this evening. Reporter: But 20 minutes later, Nathan sat back down to answer more questions. Most people would think, I'm going to die out here. And the way that I handled that was to focus on what I had to do. In order to survive. Reporter: Six weeks after Nathan Carmen was rescued, Linda carm Carmen's body still hasn't been recovered. But Nathan organizes a memorial for her in downtown Hartford, Connecticut. Conspicuously absent from the service, Nathan's three aunts. I wish very much that my whole family could have come together to pray for my mom. Reporter: Tonight, Nathan remains the focus of an intense multi-agency investigation. For "Nightline," I'm Linzie Janis in Vernon, Vermont.
This transcript has been automatically generated and may not be 100% accurate.