Transcript for Molly Martens Corbett says husband was controlling, possessive: Part 1
Hey, Molly. Reporter: Tonight in a 20/20 exclusive you'll hear for from the only people who really know what happened inside this winston-salem bedroom. Or more precisely, the only two still alive to talk about it. It's horrible. A man died. It's horrible. But a man also attacked my daughter. And a man tried to protect his daughter. Molly martens Corbett and her father Tom martens sat down with us pre-trial to tell us their version of events. What do you hope to get across today? The truth. Reporter: Molly and Tom make for a rare pair of murder defendants. She'd never broken the law; while he'd made a career enforcing it working for the FBI for 30 plus years. And yet here they are enmeshed in an international murder mystery. A why dunnit, that's made news from the mountains of north Carolina. The closing arguments happened -- Reporter: To the emerald isle. Tonight's tale begins in Tennessee, where Molly martens grew up. While Knoxville feels like home today. It is place Molly once ran from after struggles during college. I was planning on being a doctor. But really, for the first time in my life, school was hard. You know, I hadn't ever really had to study before. So that was a challenge. There were other challenges. Migraines, bouts with depression and general anxiety about her future. Molly kept it all inside. I thought maybe I would just go somewhere, you know, and figure myself out for a while. Reporter: She signs up with a nanny agency and soon receives an inquiry from a man in desperate need. Jason Corbett, who sent me messages requesting that I contact him regarding his children, it was emotional. He had lost his wife. And they were two babies. Reporter: At age 24, the one inspiring doctor throws caution to the wind and takes the job offered by the Irish widower. I arrive in Ireland, and it's beautiful. Reporter: So Jason's family say that you guys hit it off ve quickly, that you guys actually got together and slept together that first night. That's not true. But it was very quick. I was apprehensive and concerned that it was too quick. But I didn't want to call up my parents and say, "I'm uncomfortable with this, and I'm coming home. I've failed again." Reporter: The unusual relationship toggles between personal and professional. But in short order, Molly finds her place in the world. She is needed. She is wanted. She is accepted as a mother figure. It was wonderful. For me, it gave me a sense of responsibility. And it filled a void I had that made me feel like I was worth something. It was wonderful for me. Reporter: You became a mother overnight. Yes. Reporter: Were you in love with Jason? I was. Reporter: Jason Corbett has many loves. His beer, his favorite soccer team and most of all he loves his kids little jack and Sarah. After three years together, the modern family buys plane tickets to the United States. One way tickets. He loved the United States. And he thought that the opportunities for the children were significantly better. Reporter: In 2011, Irish eyes are smiling and wedding bells are ringing. Tom martens gets to walk his daughter down the aisle. On the wedding day I was exciting for them. And it was a delightful experience, and a beautiful wedding. Reporter: The kids were in the wedding? They were, of course. Uh-huh. They were flower girl and ring bearer. Reporter: The happy family soon moves to Davidson County in North Carolina and a home that's tobacco country's answer to Ireland's stone castles. Four beds, three baths and 5500 square feet for the kids to run around in. There's even enough space for the in-laws to stay overnight. It really was a dream summer. Jason was working really hard, but it was a golf course community and he was excited and golfing on his day off. From what I saw they were a loving couple. I didn't see anything that would question the relationship. Reporter: Tony turner says Jason was the king of the cul-de-sac. He was very cordial and he was loved by everyone over here. Reporter: Loved by everyone? A gentle Irish giant. He was good. Reporter: Meanwhile the kid now 6 and 4, are thriving. Jack Corbett swings a mean bat on the little league team. Young Sarah is the social butterfly, always impeccably dressed. They made friends immediately. It was like a wonderland. Reporter: Molly's got it all, except one thing. Jason is refusing to make her the children's legal mother. At that time, I felt like he was actually going to follow through with the adoption papers and that, you know, I would feel more confident about securing my rights to the children. Reporter: It never happened. Instead, Molly says Jason was growing addicted to the narcotic of domination. He was very controlling, and he was very possessive. The first if you months, you just brush it off. He loves me so much, but those things got worse. Reporter: He was uncomfortable with you socializing? He was paranoid that I would develop some feelings for someone else and or that somebody would look at me the wrong way. Reporter: He was worried you were going to leave him for another man. He was worried about a lot of things. He'd come home from buying new golf club for $500 and he'd open the fridge, and there would be a case of raspberries. And that would be it. "We can't afford raspberries." And he would throw the raspberries on the floor. Reporter: According to Molly, it is a side of Jason no one sees because to friends, he's the life of the party. The husky 260-pound irishman always has a smile on his face at neighborhood gatherings. But in private -- He would dictate what she should wear, or what she should shop for, or when she should be home, or when she should or shouldn't leave, or text her repeatedly, or engage in just various forms of, you know, controlling behavior. He would demand to see her phone, look at her computer history, that sort of thing. Reporter: Molly says the fights were escalating. And says the kids witnessed many of them this is son jack's recollection when he was later evaluated by a social worker. When he would get mad what would he do? Physically and verbally hurt my mom. Did you see him? Hurt her? A bunch of times? What did you see? Punching, hitting, pushing. Reporter: Did the physical abuse become more frequent or more extreme? Everything became more frequent and more extreme, except for the apologies. They became less frequent and less extreme. Reporter: If you're thinking Molly should pack a bag and run she says she considered it. She visited a lawyer who told her to document any abuse which she did. Making surreptitious audio recordings. I'm talking to you. Is this how you treat -- is this you just ignore me? I said I'd like to have dinner with. I'm talking to you. I shouldn't have to say it over and over. I shouldn't have to say Molly. Can you guys get out the stuff for pancakes? See? There you go again. I'm talking to you and you're still going on talking about something else. Stop fighting. Stop fighting. Reporter: But Molly is caught between a rock and a hard place. Without a legal adoption, she has no shot of custody in the event of a divorce. She says the stakes are too high to simply walk away. So you suffered because you didn't want to lose the children. I never would have left the children. You know. I couldn't imagine. Sometimes I thought, "Maybe I'm being selfish. Maybe their life would be better, you know, if they don't have to deal with this." But ultimately, I always came to the same conclusion that, you know, it wouldn't be better for them to lose a second mother. Reporter: Tom martens knows his daughter is suffering, but decides she doesn't need another man bossing her around. I wasn't going to interfere in Molly's marriage. That was Molly's marriage. Reporter: A marriage she says that only gets worse when the lights go out. Sometimes he would be angry and choking me would turn into something sexual. Or sometimes the other way around. Reporter: So sometimes he would choke you in anger, and sometimes he would choke you during sex. Did that scare you? Everything always felt so real and so scary in the moment, when it was happening. Reporter: Did you ever pass out? I did. It did always make me think of mags, his first wife, and wonder if that's what happened to her. Reporter: Mags Fitzpatrick, who suffered from asthma, died in the middle of the night. Official cause -- a heart attack. But Tom martens remembers a starling conversation he says he once had with the dead woman's father. His name was Mikey Fitzpatrick. And you know, I thought I was just making polite chitchat, polite conversation. And I said, you know, "What do you think of old Jason?" And his response was, "I think he killed my daughter." And I go, "Whoa." You know, I'm not really expecting that kind of thing. Reporter: From Ireland, the Fitzpatrick family later denied the conversation ever happened. But Molly had heard the same allegation. At some point, it was suggested that Jason may have killed his first wife, that perhaps it wasn't an accident. Yes. It definitely was. And prior to that, I mean, I knew that. Reporter: What? You know, the first time and second time and the third time and the 20th time that you are suffocated or strangled or someone holds their hand over your mouth or a pillow over your face, and you can't breathe for an extended period of time, you know, you think, "Oh, well, you know, his first wife died at 3:00 in the morning, and maybe that's going to happen to me."
This transcript has been automatically generated and may not be 100% accurate.