Waco's beloved minister, after nearly 20 years, still claims he isn't guilty of murdering his wife

Baptist minister Matt Baker was once a beloved pastor just outside Waco, Texas. Unbeknownst to his congregation, he was leading a double life that led him to commit the ultimate sin.

Baker is now serving a 65-year sentence after being found guilty in January 2010 of murdering his wife Kari Baker, in 2006.

An all-new "20/20" episode, "Dirty Little Secret, "which airs Friday, Mar. 21, at 9 p.m. on ABC and starts streaming the next day on Hulu, features John Quinones' exclusive new interviews with Texas Ranger Matt Cawthon and detective Abdon Rodriguez.

You can also get more behind-the-scenes of each week's episode by listening to "20/20: The After Show" weekly series right on your 20/20 podcast feed on Mondays, hosted by "20/20" co-anchor Deborah Roberts.

Fifteen years after his conviction, Baker still maintains his innocence, saying that his wife died of an apparent suicide.

"Kari Baker didn't kill herself. She was murdered. He was a predator," Texas Ranger Matt Cawthon -- who was involved in the case -- said in a new interview with "20/20."

In a 2010 interview with "20/20," Baker admitted to having an affair with Vanessa Bulls, a divorcée who testified at his murder trial that Baker was openly plotting his wife's death.

"I'm coming clean on the lying about Vanessa," Baker told "20/20." "I made a mistake. I'm human. I made a mistake there. I should not have ever got involved in that. I was having a tough spot in my marriage and I took the chicken way out. But I would never have hurt my wife. I never did. I never laid a hand on her, ever."

On Apr. 7, 2006, 31-year-old Kari was found dead seemingly due to an overdose of sleeping pills. There was a typed, unsigned suicide note, but her family suspected foul play.

"She wouldn't have done this, she would not have left her girls," Nancy Lanham, Kari's aunt, told "20/20." The couple had two young daughters at the time.

Baker told ABC News in 2008 that his wife's alleged downward spiral began with the tragic loss of another daughter years earlier. She was only one year old when she was diagnosed with a brain tumor and died four months later, leaving Kari devastated.

"She had been taking sleeping pills to go to sleep since Kassidy passed away," Baker said. "And started taking more and more of them. And we had discussions that 'you've got to stop doing this, this is too many, this can be dangerous.'"

The police investigation initially concluded that Kari's death was a suicide. However, her family began investigating on their own and urged police to further question her husband.

Baker passed a polygraph test arranged by his attorney. Frustrated that the police had ruled Kari's death a suicide, her mother Linda Dulin sought out an investigative team of her own.

Dulin reached out to Bill Johnston, a former federal prosecutor who was part of the team that prosecuted the 11 surviving Branch Davidians in 1993. Dulin explained to Johnston that she didn't believe her daughter took her own life.

She said she had been working on Kari's case with what she called her "Angels" -- her sisters and niece who were determined to find out the truth about what happened to Kari.

"What Charlie's Angels were bringing to us was invaluable," Johnston said. "They were this group of strong women who were not going to just let this go. They brought a series of stories about Matt's conduct at different points in his life."

Kari's family believed that Baker's affair with Vanessa Bulls was the motive behind what they believed was Kari's murder.

In September 2006, Cawthon -- in collaboration with police -- was able to get Kari's body exhumed, and an autopsy was conducted. There was still no clear cause of death, but the generic form of Ambien was discovered in her muscle tissue. The team working with Dulin discovered that Baker searched for the drug online.

The findings made it crucial to keep investigating.

"During this investigation, we got a clue from a woman who worked in a jewelry store," Cawthon said. "And said within a couple of weeks after Kari's death, Matt Baker comes into the jewelry store with a woman, and they're looking at engagement rings."

Baker denied this in his interview with "20/20," saying they had gone to the store to look at earrings for one of his daughters.

However, during the trial, Bulls testified that Baker stated that his daughters wanted to look at wedding rings for her.

Although she had been previously interviewed, she had never admitted to having an affair with Baker or that she knew anything about Kari's death. Prosecutors then decided to subpoena her to testify in front a grand jury and give her testimonial immunity, hoping she would cooperate.

To the surprise of everyone in the room, she admitted on the stand that Baker had told her he had killed Kari. It was enough for prosecutors to indict Baker for his wife's murder that same day.

At trial, Bulls said she felt controlled by Baker: "He was a complete and still is a manipulative liar who took me at my vulnerable state." Bulls also testified that she felt intimidated by Baker to keep quiet: "Don't tell anyone or you'll be just another regret"

The prosecutors' theory was that Baker had drugged Kari and then suffocated her. The testimony from Bulls corroborated that theory.

"She did explain that after he thought that he had killed her, that she'd been suffocated, that when he removed the pillow, she gave one huge gasp," prosecutor Susan Shafer said. "She was still alive, and that he immediately put the pillow back on her face and held it until he was sure that she was dead."

In January 2010, Baker went on trial for his wife's murder. After several days of testimony, it took the jury only seven hours to decide Baker's fate: guilty. It was a verdict that seemed to stun him. The next day, Baker was sentenced to 65 years in prison.

"I believe the jury made a mistake in this," Baker said in court after he was sentenced.

While Kari's family finds some relief in knowing Baker is incarcerated, they say that the guilty verdict didn't alleviate the sadness and pain he has caused them.

"She was an amazing mother and cousin," Lindsey Pick, Kari's cousin, told 20/20. "And she would fight for any of us. And, I just wanted everyone to know the truth."