Transcript for New Details From Investigators in Kayak Murder Trial
We begin with new details on the alleged murder of a young man by his fiancee. Prosecutors say interrogation tapes capture he saying she wants him dead. But here's the question this morning, will the jury ever hear those words? "20/20" anchor Elizabeth vargas is here. Angelika graswald is accused of killing Vince viafore. There is no forensic evidence and no eyewitnesses. It rests almost entirely on graswald's words during an 11-hour taped interrogation and a conversation with the detective that was not recorded and happened before she was read her rights. A state police investigator is now testifying that Angelika graswald admitted to him that she took her fiance's paddle after he capsized and held it while he begged her to call 911. The decorative also testified that during their 45-minute conversation, graswald said she felt trapped by viafore and wanted to be free of his demands of sex with him and another woman. But in graswald's 11-hour taped interrogation obtained by ABC news she repeatedly denies killing her fiance and says her desperate calls to 911 were real. I don't see him. Oh, my god. Can you see the kayak still? No, the kayak went underwater. Oh, my god. Reporter: But after hours and hours of questions punctuated with yoga breaks and hopscotch, graswald does finally utter these damning words. I wanted him dead and now he's gone. And I'm okay with it. I'm okay with that. Reporter: Graswald spoke to me exclusively from jail. Why would you say something like that? Well, they kept asking me the same questions like a hundred times. I knew that I was innocent. I was at my breaking point, I had it so I just gave them what they wanted. Reporter: At the center of the case the kayak's drain plug. Prosecutors claim graswald removed it causing the kayak to take on water and sink. Why didn't he have that plug in there. He didn't have it because I guess I had it. Reporter: This is the infamous plug. It is. That prosecutors say Angelika removed in a plot to kill Vince but this plug is, what, half an inch in diameter. Just look at how large the cockpit is where the seat is. There's a lot of water that can enter that. That's a big hole. This is a really small hole. It's a tiny hole. Our experiment shows that this kayaker had no trouble crossing the Hudson in an identical kayak with the plug removed. Did you remove the plug from Vince's kayak with an intent to kill him? No. I did not. Reporter: Angelika's attorney is under a gag order. She did not confess to killing Vince viafore and there are many reasons why he died. My compliant removing that drain plug was not one of them. Reporter: Graswald who pleaded not guilty still maintains she did not kill viafore. Did you kill Vince? No. That day? No. I loved him. I didn't do it. Reporter: Those questions will be critical, those interrogation tapes and the judge will decide after five days of hearings whether to admit them. Let's bring in Dan Abrams as well and talk about the pretrial hearing that is specifically about Angelika's tapes to police, statements that are crucial. I think even the prosecutors would concede if they can't get in these statements they have no case. This is the case. These statements and there are two sets of statements. Remember, there's first a statement that she makes before that interrogation you see on tape and that's the one where the authorities say she made even more incriminating statements and so you have a police officer basically saying, here's what she said to me. She basically told me she did it. She told me why she did it, et cetera, but then when you get her on the interrogation tape she's not saying the sorts of things that the police officer is saying she said the first time. And the detective keeps saying remember you told me, remember you told me and she's like, no. And also that statement that conversation that 45-minute conversation that was not recorded was also done before she was read her rights. I was going to ask is that the defense's argument to keep these out. That's the entire argument. Effectively what the defense is aing is they knew she was already a suspect. They were there to effectively meet her so they could interrogate her and as a result, the defense says, she should have been read her Miranda warning before she made this statement. The police are saying, no. We were there, she was not necessarily a suspect. It was only after she made those statements to us did she become a suspect and then they did read her her Miranda rights before the taped interrogation. Although it's interesting, the one witness the defense called in this hearing that just concluded testified that she went over three times on Bannerman's island to intervene because she thought the detectives were bullying Angelika so three types she went over to say, are you okay? Because that conversation that the detectives sort of portrayed as this, oh, here's -- let's have a talk on the beach. Was actually an interrogation. The drain plug quickly obviously we saw that you had an expert there showing they didn't think it would sink the kayak alone. Did she ever say why she removed it. She said she might have taken it out a few months earlier as a cat toy. Even if you buy she wanted to kill him, killing somebody like this removing that tiny drain plug would not make that kayak go down. Think about the risk to herself also in connection with it. He was a much bigger man. She was a very tiny woman. If the judge keeps it out, the case is over. Thanks, guys. Over to you. Fascinating case and Elizabeth has been on it from the start.
This transcript has been automatically generated and may not be 100% accurate.