What former FBI agent, daughter found guilty of murder say happened the night her husband was killed

— -- A former FBI agent and his daughter, who were both found guilty of murdering the daughter’s husband, told ABC News' “20/20” in a pre-trial interview that they were convinced her husband was trying to kill them and they were just defending themselves.

“I’m going to do everything that I have to do to save her life,” said Thomas Martens, 67. "And if I die trying, well … she’s my daughter. I’m not going to live with not trying. I’ll tell you that."

“I was sure that he was going to kill my father,” said Molly Corbett, 33.

Watch the exclusive interviews with Molly Corbett and Thomas Martens on ABC News "20/20" this Friday, Aug. 11 at 10 p.m. ET

Martens, a retired 31-year veteran of the FBI, and his daughter, Molly Corbett, were both found guilty Wednesday of second-degree murder for the death of Corbett’s 39-year-old husband, Jason Corbett. They have always maintained they acted in self-defense and were trying to protect each other from Jason.

They both received the same sentence of a minimum of 20 years in prison and a maximum of 25 years, but said they plan to appeal.

On Aug. 2, 2015, Thomas Martens and his wife were visiting their daughter at her home in Winston-Salem, North Carolina, where she lived with Jason Corbett and Corbett’s two children from a previous marriage, Jack, 10, and Sarah, 8. Molly Corbett said she had anticipated adopting the children, whom she had helped raise, after they were married, but Jason refused her request.

Molly Corbett said her husband had been drinking with a neighbor and took the news of the impromptu visit from his parents-in-law well. The Martens arrived around 8:30 p.m. that evening and came bearing a gift for Jack.

“I had brought over a baseball bat for Jack. He thought it was really cool to get any former baseball gear that was a hand-me-down,” Martens said.

After some pizza for dinner, everyone went to bed for the night, with Molly Corbett’s parents taking the downstairs guest room. At around 3 a.m., Molly Corbett said she was woken up by Sarah.

“She’d had a nightmare, and she came down. And the kids were not supposed to come in the room, you know, and wake up Jason. So they would just kind of stand outside the door and, like, whisper until I heard ‘em,” Molly Corbett said. “So I heard her, you know, calling me. And I got up as quietly as I could and, you know, tiptoed up to her room.”

Molly Corbett said she got Sarah back to bed and quietly returned to her room, trying not to wake her husband.

“But he woke up, and he was angry. And he wanted to know why I’d gotten up,” Molly Corbett said. “Then he was furious because Sarah had been doing this lately and, you know, she just wanted to be coddled. And she was too old for that, and I shouldn’t have gotten out of bed,” Molly Corbett said.

She said she then spoke back to her husband, emboldened by the fact that her parents were staying with them.

“I said, ‘She’s just 8. She had a nightmare. I should be allowed to go upstairs and comfort our daughter,’” Molly Corbett said. “He forgot my parents were there, and he, you know, just got angry with me.”

“I don’t know what precisely woke me up, but what I heard were loud voices and a kind of, like, thumping, like heavy feet, you know, moving around. And it was coming from the floor above me,” Martens said. “It was in the dead of the morning, and it wasn’t right. Something bad was going on.”

Molly Corbett said she wasn’t yelling, aware that her parents were just downstairs. She said Jason Corbett wanted to make her be quiet so he covered her mouth and started choking her. This had happened before, she said.

“At some point, when he stopped, I screamed, and he started again. And the next thing I remember is my dad standing in the doorway,” said Molly Corbett.

Martens had grabbed the baseball bat he brought for Jack and went upstairs, where he said he found Jason Corbett with his hands around Molly Corbett’s neck.

“He quickly moves to not having his hands around her neck, but to move her in front of him in between me … with her throat right in front of the crook of his elbow, and so he’s got her in a chokehold,” Martens said. “I said, ‘Let her go.’ And he said, ‘I'm going to kill her,’ then he starts to edge toward the master bathroom, which has a door. … And my thought was, ‘If he gets into the bathroom, if he gets that door between me and him, then she's dead. And there's nothing I can do about it.’”

Martens continued, “And so I reached around to his left, and my right -- I'm right-handed -- with the baseball bat. And I hit him in the back of the head with the baseball bat.”

The blow to the back of his head didn’t stop Jason Corbett, Martens said, and he kept dragging Molly Corbett toward the bathroom. Martens said he was able to get into the bathroom with Jason Corbett and hit him again with the baseball bat.

“I hit him hard on the back of the head again,” Martens said. “He’s still got her by the throat, but he changes tactics. He decides to come back at me, and I’m swinging the bat, and he catches the bat in his hand.”

“Jason just grabbed the bat away. It was like it was nothing. He could choke me with one hand-- with one arm and grab the bat with the other. And he was just so much stronger,” said Molly Corbett. “He grabs the bat and, you know, pushes my dad.”

At this point, Molly Corbett said she screamed at her husband to not hurt her father. “And I thought … ‘He’s going to hit my dad with the bat, and that’s it … he’s going to kill my father,’” she said.

“So I rush him and I grab the bat with two hands. And I hang on for dear life. And we're struggling' back and forth now four hands on the bat like this. And I'm trying' to hit him with the bat, and hit him with this end of the bat, and hit him with my elbow, and hit him with my fist, or anything else. But I'm going to hang onto that bat. And he goes down. And I've got the bat. And I back off,” Martens said.

Martens said he realized Jason Corbett was not going to get up. “It looks like the threat is over, so I tell Molly, ‘We need to call 911.’”

“He’s bleeding all over and I may have killed him,” Martens told the 911 operator.

The operator instructed him to begin CPR on Jason Corbett, and he passed the phone to his daughter, who then said she also tried to perform CPR on her husband.

“The whole time, I’m thinking that he’s going to sit up and start choking me again, and it was terrifying,” Molly Corbett said. “I think I was so upset that I couldn't-- but I was doing my best to perform CPR to the best of my ability... and trying to make sure that he would be OK. But I was scared the whole time.”

Police took Molly Corbett and her father to separate patrol cars and brought them in for questioning, where Molly learned for the first time that Jason Corbett was dead.

“We gave separate statements, and you know, I was told, you know, ‘Don’t worry. It looks like self-defense. It’s going to be OK,’” said Molly Corbett.

Molly Corbett and her father were photographed and told they were free to go. Deputies drove them back to the house.

In their investigation, police became focused on Molly Corbett and her father, no longer believing their self-defense story.

One of the responding officers wrote in a search warrant that, in his opinion, the crime scene was not consistent with the struggle that they had described.

During her police interview, Molly Corbett admitted that she hit her husband with a paving stone, which was discovered covered in blood. But in their pre-trial interviews with “20/20,” Molly Corbett said she didn’t want to speak about it, and her father said he can’t say for certain whether she hit her husband or not.

“I’m not going to speculate. I know I hit him,” said Martens.

Five months after Jason Corbett’s death, Molly Corbett and her father were charged with second-degree murder and voluntary manslaughter. Prosecutors called it a brutal murder, with investigators testifying that the physical evidence proved Corbett suffered fatal blows to the head after he was already down. Martens and his daughter continue to maintain it was self-defense.

Molly Corbett, who claimed Jason was an abusive husband, said it’s “horrible” that somehow because of her and her husband, her father has ended up in this situation.

“It makes me feel like, you know, I've ruined his life. That I've impacted my whole family. And it's not a good feeling,” she said.

“I didn’t murder my son-in-law," Martens added. "And I would challenge any reasonable man, much less a reasonable father, to say that this was unnecessary force. I used the force that was necessary to end the threat. That's what I was trained to do for over 31 years, end the deadly threat.”

ABC News' Lauren Effron contributed to this report