48 Hours: Do defense lawyers have a case after a confession?

HOUSTON -- On Sept. 22, 2012, Michelle Warner, mother of two young children, vanished from the Houston apartment she shared with her ex-boyfriend, Mark Castellano. Castellano told Michelle's concerned family and eventually the police that she had stormed out after the couple got into a fight.

Castellano's story made no sense to people who knew Michelle and they were adamant the 31-year-old mother would not leave her kids behind. When five days passed with no word from her, news coverage picked up and Castellano got a call from the "Dr. Phil" show, a top-rated syndicated broadcast distributed by CBS Television Distributon. In an interview, he told the seasoned TV host that same story ... Michelle was gone after a fight.

That's when homicide detectives turned up the heat and brought Castellano in for questioning. A three-hour interview led to a revelation. In a matter-of-fact tone, Castellano told the police he grabbed Michelle during that fight and broke her neck.

Castellano even directed the detectives to the shallow grave in distant West Texas where he had buried Michelle in.

With the videotaped confession and the discovery of Michelle Warner's body, it would seem that the case was closed. But it wasn't.

In spite of his confession, Castellano ended up pleading not guilty. His defense team told an alternate story about Michelle's death, that the whole thing had been a terrible accident during a fight between the two. Essentially, Castellano now says he had fallen on Michelle Warner as he defended himself and that's how she died.

Defense attorney Eric Davis told "48 Hours" it's unusual for a case to go to trial after a confession-- but in this situation, he had a plausible alternate story about how Michelle died.

Davis added that prosecutors sometimes call a statement from a suspect, a "confession"-- even if it is not. A defendant may admit to some elements of a crime, the state still has to prove each and every element of the crime beyond a reasonable doubt.

Castellano's attorneys based his defense on the "accident" story. But jurors apparently did not buy it. They took six hours to return with the guilty verdict.

However, at sentencing, there was a surprise. While Castellano could have gotten up to a life sentence, the jury gave him 27 years. That means he could be out on parole in just 13 and a half years.

Michelle's family was deeply disappointed in the sentence.

Prosecutor Tiffany Johnson told "48 Hours"she does not know how the jury came up with the 27 years but she makes an educated guess. She says if there are two opposing sides during deliberations, the jurors usually compromise and meet each other half way and this is what she believes happened in this case.

Asena Basak is a "48 Hours" field producer. "48 Hours" investigated the disappearance of Michelle Warner in the episode, "Gone." Watch the full investigation online.