Son Who Was Last Person to See His Mother Alive Speaks Out

ABC News spoke exclusively with Nathan Carman about the fishing trip that ended with his mother's disappearance.
3:34 | 02/03/17

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Transcript for Son Who Was Last Person to See His Mother Alive Speaks Out
We're back with that ABC news exclusive. A young man making national headlines, rescued at sea a week after his boat sunk. But his mother who was with him is missing. ABC's Linzie Janis sat down with him. Good morning, Linzie. Law enforcement officers are still searching for evidence of a crime on the high seas. The coast guard and even the FBI are now involved in the investigation. And just this week, the insurance company investigating Nathan Carman's claim on his lost boat concluded he intentionally did things to the boat that caused it to sink. It was a stunning discovery. A young man found drifting on a life raft missing for seven days. Nathan Carman had gone fishing with his mother Linda. But as he's pulled aboard the Chinese freighter that spotted him, she is nowhere to be found. Mom and I two people, myself and my mom were fishing at block canyon and there was a funny noise in the engine compartment. Reporter: Two days later when Nathan arrives in Boston questions are waiting for him at the dock. What happened to his mom? And was it really an accident? Nathan who has been diagnosed with asperger's syndrome pleads for privacy. My request is just to be allow to mourn naturally. Reporter: But then he decides to grant us an exclusive interview. He insists his mother's disappearance was an accident. I did not see or hear my mom. Reporter: He says their boat was taking on water and it sank so fast he simply lost sight of her. I love my mother, present tense. I love my mother. Reporter: Police are not convinced. His mother was worth millions and he's her sole heir. They executed a search want on Nathan's home looking for evidence of reckless endangerment. Boat owner Mike Iozzi told us and police that he saw Nathan hours before the trip doing something highly suspicious. It caught my eye when I saw him leaning over the back and drilling a hole, two holes in the transom of the boat. Reporter: As we discuss the allegations he reaches his breaking point. All I can say is what happened. I can't -- if other people want to -- I'm just asking you to defend yourself against these people who are saying you did something to your boat that would make it sink. That's thought true. The only way that I can speak out against them is to say what actually happened and that's what I've been trying to do. Okay. Another thing that's happened since we last got together, police searched your mom's home. Yeah. Did you know anything about that. We're done for this evening, period. We're done. We're not trading to make you uncomfortable. We're trying to give you an opportunity to answer some of these allegations. We're done here this evening. So Nathan stormed out of our interview but about 20 minutes later he came back and answered more questions. He explained what he was doing to the boat and he talked about another mysterious death in his family, Nathan's grandfather, a multimillionaire real estate developer was murdered in his bedroom three years ago and we learned Nathan is still a suspect. Why do police believe that. He's never been charged but in a 2014 search warrant police say his whereabouts on the night. Murder are unaccounted for and they tried to arrest him and a judge wouldn't sign the arrest warrant. So much more here. Yeah. Fascinating mystery. See much more of jandzlinzie's interview on "20/20." The buzziest super bowl ads.

This transcript has been automatically generated and may not be 100% accurate.

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