Cayley Mandadi, 19, was a sophomore at Trinity University when she was brought to a Texas hospital on Oct. 29, 2017. She was nearly naked, bruised and not breathing. Her sometime boyfriend Mark Howerton told doctors they'd taken ecstasy at a music festival and she passed out after consensual sex in his car. She died at the hospital.

In February 2018, authorities charged Howerton with Mandadi's murder, alleging her cause of death was blunt force trauma to the head. A year later, Howerton's trial ended with a hung jury.

A new trial was set for 2023. Mandadi's mother Alison Steele and stepfather Lawrence Baitland believed for the second trial jurors needed more information about what happened inside Howerton's car. Steele, a scientist, and Baitland, a NASA engineer, launched their own investigation.

"I needed to be able to show the jury what happened to Cayley," Baitland told "48 Hours" correspondent Peter Van Sant.

Using autopsy photos and consulting with experts, they developed a theory.

Would the couple's investigation change the course of the second trial?

CAYLEY MANDADI'S FINAL HOURS

Cayley Mandadi and Taylor Clement, best friends since ninth grade, had a streak – Snapchatting every morning.

Taylor Clement: Every single day. … She was much more of a morning person. So I would get mine first thing in the morning.

But on Monday morning, Oct. 30, 2017, there was no word from Cayley.

Taylor Clement: I didn't get a Snapchat at 8 o'clock in the morning, like I usually do. … That's actually how I knew something was really wrong.

Both good students, they bonded in science class. Clement says she was instantly drawn to Cayley.

Taylor Clement: She had like, a really infectious smile and the big eyes and like, the loving heart.

The two separated to go to different colleges. That October morning, Cayley was on Clement's mind.

Taylor Clement: As I'm working in the library, it's in the back of my head, like, I haven't heard from her.

Clement didn't know it, but just hours earlier, Cayley's mother Alison Steele and stepfather Lawrence Baitland were awakened at 4 a.m. with horrifying news.

Lawrence Baitland: "Your daughter's been involved in some incident, and she has been life-flighted to Kyle, Texas."

They raced from their home in Houston to the hospital, praying Cayley was OK.

Peter Van Sant: What was it like when you entered that room? What did you see?

Alison Steele: I saw my daughter's body smashed.

Lawrence Baitland: I could see that she was on a ventilator … and my heart just stopped. And I knew that it was bad.

Within hours, Cayley's parents were told there was no hope for recovery. Their daughter, just 19 years old, once so full of life, was soon declared brain dead. Baitland and Steele want the world to know what happened to her.

Alison Steele: All that potential had been destroyed. And not knowing how it happened. Or how it was even possible.

They invited Cayley's friends to see her one last time. Clement sat with Steele as she held Cayley's hand and prayed.

Taylor Clement: I remember her repeating over and over, "this has to be for something, this has to be for something."

Cayley had previously requested that her organs be donated to help others. As she was taken to surgery for that procedure, her parents said goodbye.

Alison Steele: It was very emotional. … But, of course, we didn't want to let her go. But this is what had to be.

Just days before, things seemed to be going well for Cayley. A sophomore communications major at Trinity University in San Antonio, she had joined a sorority and was a cheerleader. And she'd met a boy.

Alison Steele: Cayley at the time was very much in love with the only serious boyfriend she had ever had. His name was Jett Birchum.

Birchum was a Trinity football player and fraternity brother. Cayley dated him freshman year, but they broke up.

Alison Steele: What she told me was, "I don't know that he wants a serious relationship." And she did.

Taylor Clement: I think that part of her life got very murky for her very fast.

There was another man in Cayley's life. Mark Howerton was 22 years old and had been a star high school baseball player.

Howerton lived in Houston but was often on Trinity's campus visiting friends. That's where he met Cayley.

John Hunter: I think that they're both very beautiful people and I think that that was the primary attraction.

John Hunter is Howerton's lawyer.

John Hunter: I think that there were problems with her relationship with Jett. … And Mark was offering an alternative to that.

Both Jett and Mark knew Cayley was seeing the other man and neither, friends say, liked the competition. Then, just one month into this new relationship, Mark Howerton rushes Cayley to a small rural hospital in Luling, Texas.

While medical staff tended to Cayley, police officers interviewed Howerton in the quietest place they could find – the hospital chapel. It was recorded on the officer's bodycam.

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.

MARK HOWERTON GOES ON TRIAL FOR THE MURDER OF CAYLEY MANDADI

In December 2019, two years after Cayley Mandadi's disturbing demise, Mark Howerton is finally brought to trial for allegedly kidnapping, assaulting and murdering her.

JUDGE:  How do you plead, Mr. Howerton.

MARK HOWERTON:  Not guilty, your honor.

Peter Van Sant: What was it like to see Mark Howerton in the courtroom?

Alison Steele: Like everybody else, I'd try not to look at him too much.

The trial begins with prosecutor Alessandra Cranshaw's opening statement.

ALESSANDRA CRANSHAW (in court): I think the best way to tell this story … is to start where Cayley Mandadi's life ended. And that's in the car of this defendant.

Prosecutors allege that Howerton forcefully escorted Cayley from the Mala Luna Music Festival to his car. He then drove her to a parking lot where he sexually assaulted and beat her, causing a fatal brain bleed. Cranshaw presented photos showing Cayley's condition about 18 hours after she arrived at the hospital.

ALESSANDRA CRANSHAW (in court): Cayley is covered with bruises from head to toe … I believe that you will have no reservations about finding this defendant guilty of the offense of murder.

JOHN HUNTER (in court):Things are not always as they seem.

In his opening statement, defense attorney John Hunter attacks the very foundation of the prosecution's case.

JOHN HUNTER (in court): Mark Howerton did not cause Cayley's death.

JOHN HUNTER (in court): No one saw anything that transpired between Mark Howerton and Cayley Mandadi that evening on the 29th. There are no eyewitnesses.

The state begins its case with paramedic Sharyl Lane, who was driving an ambulance up to the Luling hospital that night when a vehicle suddenly appeared behind her.

SHARYL LANE (in court): There's a black car behind us honking and flashing the lights. … Before I even get out, I see a gentleman that's hollering and screaming. I open the door. He's telling me that — his girlfriend is not breathing. And he needs help.

When Lane looked inside Howerton's car, she saw a disturbing scene.

SHARYL LANE (in court): She was exposed … And her pants and her clothes … were in the floorboard by her feet. … She … had bruises on her, just multiple bruises that I just remember seeing. … that's when I started CPR.

But Hunter says not everyone at the hospital believed Cayley had been attacked.

John Hunter:  The treating physician at the Luling hospital — approached this as a drug overdose.

Peter Van Sant: Wasn't that because Mark had told them he thought she had overdosed?

John Hunter: Correct. But the symptoms she was presenting were consistent with that patient history.

Peter Van Sant: Was this woman beaten or not?

John Hunter: No.

Hunter insists the photos showing bruising on Cayley don't point to foul play.

John Hunter: By the time that the sexual assault nurse … photographs her … she has been resuscitated. Six times, I believe. Maybe eight. … She's had a chest tube inserted. She's had IVs placed on both arms. … She's been worked over by those physicians at the Luling hospital in a traumatic way.

While the medical team fought for Cayley's life, police interviewed and photographed Mark Howerton. Police officer Chris Adams.

OFFICER CHRIS ADAMS: While I was taking photographs of him. I noticed … the tops of both of his hands.

DAVID LUNAN | Prosecutor: Do they look red to you?

OFFICER CHRIS ADAMS: Yes, sir.

DAVID LUNAN: Unusually red?

OFFICER CHRIS ADAMS: Yes, sir.

Evidence, investigators believe, that Howerton did hit Cayley.

DEPUTY CALENTINE (to Howerton on bodycam): Those actually do look like they have scabs already on them.

The Mark Howerton arrested after Cayley's death was an enormously muscled, intimidating figure. Cayley's friends say he used steroids.

Christy Jack: … even in such a confined space. … it would've been relatively easy for him to cause that kind of damage to her head.

Howerton initially cooperated with investigators, allowing police to search his Mercedes, where they found a gun and marijuana.

John Hunter: If he was worried about himself, he would have told the police to get a warrant.

Instead, Hunter says, Howerton was much more concerned about Cayley.

MARK HOWERTON (police interview): Can I find out an update on her, please? Is there any way?

The only person who could counter Howerton's story was dead. Investigators hoped Cayley's remains might speak on her behalf. Dr. Suzanna Dana performed the autopsy and said the evidence shows Cayley was beaten to death.

DR. SUZANNA DANA (in court): She had a number of what I call blunt force injuries to her face and her head. …

ALESSANDRA CRANSHAW: And what is this that we see behind the ear?

DR. SUZANNA DANA:  That's a bruise. That's a contusion.

But during cross-examination, Hunter gets Dana to admit that at least one Cayley's injuries was caused by her medical care.

JOHN HUNTER (in court): So CPR can cause a lot of different injuries, not just simply superficial ones. Right

DR. SUZANNA DANA: Yes. 

JOHN HUNTER: In this case, a rib was broken. Is that correct.

DR. SUZANNA DANA:  I believe so. Yes.

Whatever the jury thinks about how Cayley died, they're about to learn much more about her troubled love life with Howerton.

A DEFENSE WITNESS MAKES A STUNNING CLAIM

In the agonizing final hours of Cayley Mandadi's life, her friends gathered to say goodbye. Cayley's childhood friend Taylor Clement met her college friends for the first time. They bonded talking about Cayley.

Taylor Clement: Even in that situation, she was bringing people close that would've never met before.

Some of those friends now nervously wait to testify in Howerton's murder trial.

ALESSANDRA CRANSHAW (in court): Do you recognize this person?

MORGAN SAMPSON: I do.

ALESSANDRA CRANSHAW: Who is this individual?

MORGAN SAMPSON (voice breaking): It's Cayley.

Her roommate, Morgan Sampson, told of a stormy history between Cayley and Howerton, including one particularly disturbing incident just a few weeks before Cayley's death. She was planning to go to a party and says Howerton got upset when she invited Cayley to come along.

MORGAN SAMPSON (in court): Mark and Cayley were out on the balcony. And he had thrown her up against the brick wall —

Cayley ended up going to that party against Howerton's wishes. Soon, neighbors reported hearing loud banging in Cayley's room. Campus police officer Roderick Lewis was dispatched to investigate.

OFFICER LEWIS (in court): When I arrived at the location … a male subject was exiting the room.  

OFFICER LEWIS (bodycam video): And so you're in the room just waiting on her while she's at a party?

MARK HOWERTON: Exactly.

OFFICER LEWIS: OK.

OFFICER LEWIS: ... You mind if I take a look in the room?

MARK HOWERTON: Man, no ...

OFFICER LEWIS: I would like you to step out here with my partner right now.

When Lewis did go into Cayley's room, he found the glass door to the balcony was cracked, and Cayley's clothes were scattered in the trees. Friends say Howerton later smashed Cayley's laptop on the street. The university barred him from campus.

Cayley told her ex-boyfriend, Jett Birchum, that she wanted to get back with him.

Christy Jack: She was really torn between two different people and both of whom were vying for her affection.

Birchum told the jurors it all came to a head at the music festival.

JETT BIRCHUM (in court): She said she wanted to break up with him at Mala Luna 'cause there'd be plenty of witnesses and people around.

ALESSANDRA CRANSHAW: Did she seem worried about breaking up with him?

JETT BIRCHUM: Yes.

Howerton picked Cayley up to go to the music festival. But first they pulled into a nearby carwash to take MDMA, or molly. Howerton said they had more once they got to the festival.

John Hunter: We're talking about massive doses, well above what would be necessary for it to affect — be effective — to gain the experience.

When Cayley didn't show up on campus that night, her friends started calling her. Howerton answered Cayley's phone.

MORGAN SAMPSON (in court): I was like, "Have you seen Cayley?" Or, "Have you heard from her? Like are you with her?" And he said, "She can't talk right now," and then hung up the phone.

Panicked, they began searching for Cayley. Jett Birchum, who also attended the festival, says he saw Mark and Cayley move towards the exit… seemingly against Cayley's wishes.

JETT BIRCHUM (in court): Cayley looked like she was tryin' to create space and kinda just get away.

ALESSANDRA CRANSHAW: And then what do you see him do?

JETT BIRCHUM: As she's tryin' to step away, I see him reach out his right arm and hook it around her shoulder and pull her in closer. And then turn and walk away.

Peter Van Sant: This is important testimony.

Christy Jack: It's important testimony because it's showing that she's being … being taken someplace against her will.

Defense attorney John Hunter hammers him on cross-examination.

Christy Jack: It immediately falls apart almost from the very first question.

In his questioning, Hunter confronts Birchum revealing how he initially misled investigators about what he had seen.

JOHN HUNTER (in court):  You advised the police that you saw Mark physically pick her up and place her in his car, isn't that correct?

JETT BIRCHUM:  Yes, sir.

JOHN HUNTER: And it differs from the testimony that you gave today, right?

JETT BIRCHUM: Yes, sir. … I did not see him put her in the car.

Birchum admits he exaggerated to police. But why? Prosecutors say he was worried about Cayley's safety and wanted to convince cops to search for her. Hunter then challenges Birchum regarding inconsistencies in his testimony to a grand jury about the timeline at the music festival.

JOHN HUNTER (in court): Would it surprise you to learn that the phone call you placed to Cayley Mandadi's phone was made at 7 o'clock?

JETT BIRCHUM: No, sir.

In an effort to undermine Birchum's credibility, he is asked about an unrelated drug charge.

JOHN HUNTER (in court): You were on probation at the time, were you not, Mr. Birchum?

JETT BIRCHUM: Correct.

In 47 minutes of brutal questioning, Birchum "takes the fifth" 30 times, with his lawyer by his side.

JETT BIRCHUM:  … provided by the Fifth Amendment.

The damage was done.

Peter Van Sant: Tell me the impact of him saying over and over that he's taking the fifth?

John Hunter: The impact I think can't be understated. It's — it's a huge thing to see somebody do that.

Hunter has a star witness of his own — Dr. William Anderson, a forensic pathologist and former medical examiner who reviewed Cayley's autopsy and records for the defense.

DR. WILLIAM ANDERSON (in court):  So you look at the autopsy you're going to see—you're going to see what looked like bruises.

Anderson presents the crux of Hunter's case: that Cayley's bruising was caused in part by resuscitation efforts at the hospital, and the organ donation process.

Anderson goes a step further, pointing to a thin line in a photo of Cayley's skull.

DR. WILLIAM ANDERSON (in court, pointing to autopsy photo): Starting here we have this jagged line. So, like I said, it's like a crack in an eggshell, and that is a skull fracture.

A skull fracture, which Hunter says could indicate that Cayley's brain bleed was caused by a fall.

Peter Van Sant: She was in a car.

John Hunter: Well, she was in a car for portions of … that day. … Hematomas don't kill people instantaneously.

CHRISTY JACK: I don't think that you can overstate the importance of his testimony from a defense perspective. … It creates the impression that all of these injuries occurred by every other means, but Mark Howerton.

Once the defense rests, prosecutors bring the medical examiner back to challenge Dr. Anderson's testimony.

ALESSANDRA CRANSHAW: Is that a skull fracture?

DR. SUZANNA DANA:  No, it's not.

Christy Jack: The faint line that Dr. Anderson was referring to came from a picture. … I didn't believe it. I thought it was reckless for him to have that opinion so definitively with only a picture.

The jurors must now make a momentous decision. Did Mark Howerton kill Cayley?

PARENTS LAUNCH THEIR OWN INVESTIGATION: "CAYLEY GUIDED US"

After just 10 hours of deliberating, the jury in the Mark Howerton murder trial sends a note to the judge. There will be no verdict.

JUDGE RAYMOND ANGELINI (reading note): "After careful deliberation and discussion, unfortunately we cannot come to a unanimous decision." Ladies and gentlemen of the jury, I am declaring a mistrial.

Peter Van Sant: Mistrial. Are you happy? Is that in a way, a victory for you?

John Hunter: Any time that your client doesn't go to prison, it's a victory.

Alison Steele (sighs): It was a little bit disheartening. But at the same time, I was mentally prepared for that outcome.

Prosecutors David Lunan and Alessandra Cranshaw were disheartened, too.

Peter Van Sant: Did you feel as though you'd let the family down?

David Lunan: Well, sure.

Alessandra Cranshaw: Yeah.

David Lunan: Absolutely.

But it doesn't end there. The prosecutors plan to retry the case and get busy preparing for round two, starting with the defense star witness, medical examiner Dr. William Anderson.

Alessandra Cranshaw: I was not prepared for Dr. Anderson to testify, uh — to what he testified to.

They'd been stunned by his claims that Cayley had a skull fracture, and her bruising was caused in part by the organ donation process.

Peter Van Sant: So, do you find that this notion that organ harvesting caused these bruises to be absurd?

David Lunan: Absurd. Absurd. In this instance, it's absurd.

While the prosecutors work to address those claims, defense attorney John Hunter files a motion to get the whole case dismissed, based in part on what he says were Jett Birchum's lies.

John Hunter: The knowing use of false evidence by the prosecution undermines our entire faith and confidence in the judicial system. … If that can happen, then there's no point in even having a trial.

Cayley's mom Alison Steele and stepdad Lawrence Baitland sat through the first trial and concluded jurors needed more information about what happened inside Mark Howerton's car.

Peter Van Sant: You became investigators?

Alison Steele: We did.

Lawrence Baitland: I knew we needed to show what happened in that car. … The jury needed to know how it happened.

She's a scientist, he's a NASA engineer. They got to work.

Lawrence Baitland: I spent hours and hours, um, studying autopsy photos, and I really fixated on one of the autopsy photos that showed an impact on Cayley's head, you know, right in front of her ear. 

The photo of a small dot above Cayley's right ear. Other photos show deep bruising above Cayley's left ear, like the one described by the medical examiner in the first trial.

ALESSANDRA CRANSHAW: And what is this that we see behind the ear?

DR. SUZANNA DANA:  That's a bruise. That's a contusion.

Cayley's parents believe that when taken together, the photos tell a story.

Alison Steele: We believe that of all of the injuries, the blows landed on her … the fatal one occurred when he reached from his driver's seat, hit her in the left ear and drove her head into the window and onto the lock button of the car.

In fact, in a recorded audio interview with police the day Cayley died, Howerton admitted he had previously done just that.

RANGER RAYMOND BENOIST (police interview): Did you ever push Cayley's head up against the window?

MARK HOWERTON: I pushed her, and she hit the window one time. That was over a week ago, yes.

Christy Jack: So, in many respects, it was like, same song, second verse.

They'd need proof to convince a jury. Cayley's parents headed to a used car lot, where Steele posed in a car similar to the Mercedes Howerton drove that night.

Lawrence Baitland: I'd photographed her head in different positions while I'm holding the autopsy images, trying to see if they match up with the door and it's a near perfect match.

Baitland decided to go a step further.

Lawrence Baitland: So, then I loaded this 3D modeling program.

Building a 3D model of Cayley's head.  

Lawrence Baitland: … first thing you can do is you can create a solid out of that 3D mesh. And then … you can project an image onto it.

And bringing them closer to having actual proof.

Lawrence Baitland: So this gave me the confidence to go to the next step, which was to seek out the killer's car.

Mark Howerton sold the car in 2018. Baitland tracked down the new owner and bought the car.

Peter Van Sant: What'd you think of that?

David Lunan: Uh, well, this is new. This is, uh, not something I've been, uh, accustomed to hearing, uh, in other cases.

Peter Van Sant: Show me what you believe went down.

Lawrence Baitland: Sure.

Peter Van Sant: Let's go over to the passenger side…

The car Cayley was fatally injured in is sitting in her parent's driveway. Baitland says driving it home was a haunting experience.

Lawrence Baitland: It's torturous. Um, but it's also, you know, she was showing us what happened, you know. She was guiding us to this car.

Peter Van Sant (standing outside of the car): You believe that he struck her. When he did, she came over to this knob, her hit — her head, hit it. Now you notice when you push it down, it doesn't go flush.

Lawrence Baitland:  Right. … This will not retract fully, no matter how hard you hit it.

Cayley's parents decided to make a video to demonstrate their theory in court.

Christy Jack: They went and found two actors, a male and a female to dress the parts. They were, um, similar size, similar weight.

Lawrence Baitland: If we did the reconstruction ourselves, it would be considered biased and probably thrown out or at least discredited.

So they hired a private investigator to produce the demonstration. They didn't even look at it in case they were called to testify about it.

The video is simple: showing three angles of what Cayley's parents and their experts believe happened.

Peter Van Sant: Would showing a jury a video like this potentially help the prosecution's case. If it — if it's allowed?

Christy Jack:  Jurors are very visual. … It makes it easier for them.

Christy Jack: It shows how it can happen. And it answers a number of the questions that the jury had in the first trial.

Hunter says however Cayley got that dot above her ear, it's all speculation.

John Hunter: It could be from something at the hospital, it could have been from something before she got in Mark's car. … Could it be that that is from a locking thing on the door? I mean, sure. It could also be that a space alien came and put a little, you know, mark on her.

John Hunter: I almost feel like it's not worth discussing because it's so unscientific.

Years go by. Alison worked to pass the Texas Clear Alert Bill. It enables law enforcement to quickly initiate searches when people aged 18 to 64 go missing. In 2021, a district court in Texas denied John Hunter's motion to dismiss the case, ruling there was no evidence of prosecutorial misconduct relating to Jett Birchum's testimony. Hunter appealed that ruling.

Alison Steele:  It was horrible, just waiting and not knowing what was gonna happen.

Hunter filed appeals all the way to the United States Supreme Court, which declined to hear the case.

John Hunter: We lost in every court we could take it to.

And so, more than five years after Cayley's death, a date for a second trial is set. May 23, 2023.

Peter Van Sant: What's at stake here? … Are feelings, are emotions running high?

Christy Jack: I don't think the stakes could be any higher.

JUSTICE FOR CAYLEY

In May 2023, Mark Howerton is about to go on trial for the second time for the murder of Cayley Mandadi.

Taylor Clement: I felt confident that this was her time.

In the five plus years since Cayley's death, Howerton was active on social media, posting things like, "Come @ me legally, I'll beat you  ..." —  showing little remorse.

MARK HOWERTON (social video post): Wow. 75% of people think I'm guilty. That just means 75% of people in the world…can suck my d***.

Christy Jack: He was living large and bragging about having beaten the state of Texas in a court of law.

Howerton's attorneys John Hunter and Hallie Pease say acting out on social media was his way of coping with the pressure of being falsely accused of murder.

Hallie Pease: He was using social media as an outlet for — to relieve some of that tension.

During the first trial, jurors had only one charge to consider – felony murder. This time around, lawyers add more charges, including aggravated assault and criminally negligent homicide. Then came time to consider the video Cayley's parents had made.

Peter Van Sant: Did you want the jurors to see that video?

David Lunan: I did, initially.

Presenting the video came with risks, including the risk that Hunter's cross-examination about it might undermine the whole case.

Christy Jack: The judge said I'm gonna let it in. But if so … then the parents are now witnesses and they will no longer be allowed to sit in the trial. … And the prosecution went and talked to the parents, and they decided to withdraw the exhibit.

On May 23, trial two begins — this one without television cameras. Alessandra Cranshaw's opening argument is nearly the same.

Alessandra Cranshaw: I basically told the jury that … the best way to explain this case … is to start where Cayley Mandadi's life ended and that's in the car with this defendant.

But there is a big change to the witness list: no Jett Birchum.

Christy Jack: There was nothing to be gained.

And prosecutors add a witness - an expert on domestic violence who told jurors Howerton was a textbook abuser, who isolated and manipulated Cayley, and ultimately became enraged and lost control.

A pattern particularly difficult to handle at just 19 years of age.

Christy Jack: I think she had no idea, the dangerous predicament she was in.

John Hunter wants jurors to give Howerton the benefit of the doubt.

John Hunter: The … presentation of how Cayley looks in the hospital and how she looked at the autopsy does not mean that she was beaten.

His case, this time around, relies heavily on the testimony of a pharmacologist who said the high levels of MDMA, or ecstasy, in Cayley's system could have caused her brain bleed.

John Hunter: I have always felt that that was the major precipitating reason for her death.

But prosecutors and Cayley's family say Hunter grossly exaggerated the danger of MDMA.

Alison Steele: There's never been a single documented case anywhere of MDMA causing a subdural hematoma.

Peter Van Sant: What do you say to that? 

John Hunter: Well, first of all, they haven't done much research on this at all.

Hunter plans one final witness — the former medical examiner whose testimony about organ donation and a possible skull fracture caused so much damage to the prosecution in the first trial — Dr. William Anderson.

This time, prosecutors are ready for him, with a new witness, a respected medical examiner who told them Anderson's claims in the first trial were irresponsible.

Alessandra Cranshaw: They realized that we were gonna be more than prepared to shoot down all those claims that he had made in the first trial.

John Hunter: I didn't need to have a fight about the skull fracture.

So the defense rests without Anderson.

Lawrence Baitland: It was a shock. … You could hear a gasp in the courtroom …

Now, a second set of 12 people will determine Mark Howerton's fate. It's an agonizing second wait for Cayley's parents.

Christy Jack: This is the moment of truth; will their daughter get justice?

Agonizing for John Hunter, too. He'd spent nearly six years working to keep Howerton free.

Peter Van Sant: So the jury comes back in, what do you hear?

John Hunter: The two most beautiful words in the English language, not guilty.

Peter Van Sant: Not guilty of murder. 

Alison Steele: My heart sank when we heard not guilty on the murder.

Christy Jack:  … they're thinking, dear God, this is the final nightmare. He's gonna walk outtalk the courtroom.

But there was more. Howerton was convicted of aggravated assault causing serious bodily injury.

David Lunan: Everyone was hugging each other and expressing their relief that Mark Howerton was going to prison.

Alison Steele: I turned to my left and eight sheriff officers had assembled to take him to jail. Eight. So, that was their way of saying, "we're here for you." (cries) So, that was a — that was a nice moment to see that.

Howerton was sentenced to the maximum – 20 years in prison. To Cayley's family, it's not nearly enough.

Lawrence Baitland: Mark Howerton is a monster, and he does what a monster does.

For Cayley's friend Taylor Clement, the wound remains raw.

Taylor Clement: I miss my best friend.

Baitland and Steele say they have no regrets about spending time and money on a video the jury never got to see.

Lawrence Baitland: It helped us learn what happened. … We would do it over again if we had to.

"48 Hours" showed Steele that video for very first time.

Alison Steele: It's hard to see because that's most likely the blow that killed her.

Cayley's mom, who prayed something good would come from her daughter's death, continues her fight to help other victims of violence.

Alison Steele: What I would like is for her sacrifice … to help other people. That's what she would want. I have absolutely no doubt about that.

Mark Howerton will be eligible for parole in 2033.

Produced by Mary Ann Rotondi  and Chris Young Ritzen. Jenna Jackson and Ryan N. Smith are the development producers. Mike McHugh is the producer/editor. Michael Loftus is the associate producer. Gregory F. McLaughlin, George Baluzy and Michelle Harris are editors. Peter Schweitzer is the senior producer. Nancy Kramer is the executive story editor. Judy Tygard is the executive producer.

In:
• 48 Hours
• Texas

Peter Van Sant

Correspondent, "48 Hours"