DEPUTY CALENTINE (to Howerton/bodycam video): So tell me, where were y'all at today?

Howerton told officers that he and Cayley went to the Mala Luna Music Festival. There they drank alcohol and took MDMA – a drug known as Molly or ecstasy.

Christy Jack is an attorney based in Fort Worth. "48 Hours" asked her to look at files in this case. She says things took a troubling turn at the music festival.

Christy Jack: They saw Jett Birchum at some point and began to argue.

Howerton says sometime after 4 p.m., he and Cayley left in his Mercedes — still arguing after Cayley told him she still had feelings for Jett.

MARK HOWERTON (police interview): I was pretty much saying … You need to get over this dude. He ain't worth your time. ... your friends are fake as — I was just telling her all this s*** … I was trying to get it through her head …

Howerton said they then stopped in a parking lot where they had make-up sex.

MARK HOWERTON (police interview): We were having sex. I choked her. But it wasn't like I was killing her. It wasn't — it wasn't like that.

Rough, said Howerton, but consensual.

MARK HOWERTON (police interview): 500 percent consensual.

MARK HOWERTON (police interview): After we had sex like she still talked to me for five, six, minutes afterwards …That's when she was like, "I'm not feeling good" but then she just passed out.

They got back on the road. Hours passed. Howerton says at some point Cayley stopped breathing. And he tried to resuscitate her.

Christy Jack: And at 10:30 at night … he sees a sign … that indicates a hospital … at the next exit. He pulls off.

Emergency medical workers told investigators they instantly knew Cayley was in bad shape. She was nearly naked, bruised, and had no pulse.

Christy Jack: You have a — a paramedic or a nurse who says these bruises were on her when she arrived at the hospital in Luling.

MARK HOWERTON (police interview): Why wasn't she responding? (puts his hands in his lap)

Howerton told the police officer Cayley already had bruises on her legs earlier in the day.

MARK HOWERTON (police interview): She bruises easy. I — she — look, seriously, she gets drunk, and she falls over and she bruises.

Police interviewed Howerton several times, but he was not charged with any crime related to Cayley's death. Then, three months later, her autopsy report was released. The medical examiner ruled that Cayley had died from blunt force face and head trauma; it was ruled a homicide. Mark Howerton was charged with murder. He pleaded not guilty and was released on bail.

John Hunter: There were large amounts of drugs taken over the course of this weekend. Mr. Howerton brought the deceased to a hospital, which is something you don't typically see.

Peter Van Sant: Murderers don't usually do that.

John Hunter: No.

John Hunter: The idea that it was open and shut is a mistake.

As the trial begins, John Hunter says he has evidence that will surprise the jury.