Produced by James Stolz and Jaime Hellman

[This story first aired on March 16. It was updated on December 21.]

After three murders with chillingly similar scenarios occurred in Austin, Texas, police began to wonder if they were chasing a serial killer. But whomever committed the crimes left no evidence, no DNA. Cops went into a frantic investigation. The hunt for clues changed when a man testing out a new thermal imaging scope recorded the nighttime movements of a stranger the night of the first murder. As "48 Hours" correspondent Maureen Maher reports, it was evidence cops never expected – and would help them break the case.

A SHOCKING CRIME

These days in Texas, it seems like all roads lead to Austin. The sleek skyline of the Lone Star capitol---glittering. A boom town that welcomes newcomers, chasing dreams. It's a city charged with life -- an unlikely place to find tragedy as dark as the death of a dreamer, like Kathy Blair.

Kirsten Mathieson | Kathy Blair's sister: She loved life … you loved being around her.

Maureen Maher: What were the things that were important to her?

Kirsten Mathieson: She had, I think, a sense of justice, right and wrong.

Kirsten Mathieson's is Kathy's younger sister.

Kirsten Mathieson: Kathy was larger than life. … she's my big sister. … I have always thought everything Kathy did was amazing.

And even as kids in California, it was clear, Kathy had a passion.

Kirsten Mathieson: She was always singing. … She had a God given talent, which was her voice.

Kathy Blair's passion for music led her to a life in Austin, Texas, where she was a well-liked choir director. Kirsten Mathieson

It was music that led Kathy to Austin.

Kirsten Mathiesen: Kathy … went to U.T. Austin to get her master's degree in vocal performance. She loved Austin.

But love didn't always work out for Kathy; she'd divorced twice. Still, her affair with Austin held firm. And by 2013, Kathy was renting a house on a quiet street on Tamarack Trail.

Maureen Maher: So, this really felt like home to her?

Kirsten Mathieson: It was home. She loved the people here. She loved the vibe.

In a city known around the world for music, Kathy Blair fit right in. She had melody and rhythm in her soul. But you wouldn't find her singing the blues on Austin's famed 6th Street. Instead, she chose a more spiritual stage for her talents.

Barbara Szalay: Christian Choral Society was a positive social setting. The kids were kind to each other.

Barbara Szalay's daughter was one of hundreds of students touched by Kathy's talent as a choir director and teacher.

Barbara Szalay: I think she lived, breathed, ate, slept music.

Szalay, along with Kathy's student Kristin DeGroot, met with "48 Hours" to share their memories of Kathy.

Kristin DeGroot: She was so kind, um, just believed in me wholeheartedly, which was something I really needed.

And for DeGroot, Kathy was a role model.

Kristin DeGroot: She and I were the same. That music needed to be in our lives or we would die.

Maureen Maher: It's like she gave you permission.

Kristin DeGroot: Yeah. … It's OK for this to be the way that you love the world.

Kirsten Mathiesen: She was their teacher and their mentor. …One of her friends called it the Kathy Nation.

Maureen Maher: The Kathy Nation.

Kirsten Mathiesen: The Kathy Nation.

It was Dec. 6, 2014. Kathy's son Joe was staying with her while waiting for his assignment from the Navy. After a night out, he came home to Tamarack Trail. What he found was shattering and echoed across that Kathy Nation:

JOE HARGIS TO 911: Hey, I need you to come to my house right away. 911 OPERATOR: What's happening there, sir? JOE HARGIS: I think my mom is dead. There's a lot of blood. Think someone broke in and killed my mom. 911 OPERATOR: Joe, what's your mom's name? JOE HARGIS: Her name is Kathy Blair.

Det. Derek Israel: This is a case that sticks with you throughout your life.

Detective Derek Israel was working in the Austin Police Department's homicide unit at the time of the murder.

Det. Derek Israel: This case was clearly different, really, right from the -- right from the get go.

Starting with the location.

Det. Derek Israel: Oh, this is a nice neighborhood … This is a place where people I think feel safe.

Up until this case -- which would frighten and chill Austin, and shock veteran detectives Scanlon and Israel.

Det. Derek Israel: One of the first things I thought of – I'm like, well, "Why this house"?

Maureen Maher: Yeah, "Why this house?"

Det. Derek Israel: There's nothing about this house that makes this house stand out from all the other ones. …This is the living room right here.

Then, in Kathy Blair's bedroom ...

Empty jewelry drawers sit stacked on the floor of Kathy Blair's bedroom Evidence photo

Det. Derek Israel: There's a full-size jewelry case right here with large drawers. … All the drawers have been pulled out, and they're stacked up. … And they're empty. So it's like someone dumped 'em out and then put 'em in a pile right there.

Maureen Maher: Someone who had time to do that.

Det. Derek Israel: Correct.

A jewel thief who had time, because Kathy Blair was already dead.

Det. Derek Israel: This murder started right here on the bed.

Kathy Blair, 53, lay alone, asleep in her own bed. She awoke to the ultimate nightmare.

Det. Derek Israel: Probably having no idea what's going on, why this is happening, who this person is.

Maureen Maher: You are describing a horrific, violent fight to the death

Det. Derek Israel: Yeah, Kathy Blair fought like hell.

Choked, stabbed, and finally, slashed across the neck.

Detective Kerry Scanlon was the lead investigator on the case.

Det. Kerry Scanlon: The wound is a fatal wound. But she still has time, you know, to put up that fight.

Det. Derek Israel: She – she fought for her life. … Kathy's here [points to her bed] … and there's blood all around her.

There was so much blood, that it formed the timeline of a murder.

Det. Derek Israel: There's a light switch, and on that light switch we saw blood, like a blood swipe … That told us that the perpetrator had come in here after the murder and -- and switched that light on. … There are more blood swipes on these drawers. … That tells me … the murder of Kathy Blair occurs before these drawers were removed.

Word soon spread across Austin and across the Kathy Nation.

Kristen DeGroot: And I said, "No -- that's not what happened. That cannot possibly be what happened."

Maureen Maher: Was Kathy Blair the kind of woman who might have an enemy who would do that?

Barbara Szalay: No. … She didn't have a malicious bone in her body.

Derek Israel: Why does someone come in here and -- and murder someone in order to steal a little bit of jewelry? You know, it doesn't make sense.

It would be the first in a hideous series of senseless events.

Kirsten Mathieson: It's just one of those moments where you're in disbelief. You think you're living in a dream. This does not happen.

But it did happen. And the killer left almost no evidence. No DNA.

Maureen Maher: No fingerprints … and no blood from the killer?

Det. Derek Israel: No.

Israel and Scanlon would need all their street smarts and then some. Because just nine days later …

Det. Derek Israel: I get notified that there has been another murder.

THE SEARCH FOR A SUSPECT

For Austin detectives Israel and Scanlon, images of Kathy Blair's death were harrowing.

Det. Derek Israel: Attacked in the middle of the night … And it was a really horrific scene.

But there was no physical evidence from a killer who made virtually no mistakes. That meant there was no clear suspect.

Then, suddenly, the search for a suspect changed in a way no one could imagine.

It was around 1:30 a.m. on the night that Kathy Blair died. One of her neighbors was out for a late-night walk. What he saw and what he did would give the detectives their first big break in the case.

Rob Leef: I was out testing a thermal scope … I needed to get some video of some deer. And we've got some deer … up and down the street.

Rob Leef lived a few short blocks from Kathy Blair.

Rob Leef: So, it's a thermal scope, so it picks up a heat signature.

Maureen Maher [looking through the scope]: Wow, it's like daytime.

Except it was the dead of night. And Leef was only looking for deer.

Rob Leef: I saw the headlights of a car coming up. … I saw the car pull up and park, and --

Maureen Maher: On this street?



Rob Leef: On this street.

Leef kept recording.

Rob Leef: I zoomed in with the scope. And by the time I had zoomed in, someone had gotten out and walked over to the sidewalk.

You can flip the setting to red to see the image more clearly.

Maureen Maher: Did you focus in on the car?

Rob Leef: I first focused in on the person. … And he turns left on Tamarack.

Kathy Blair's street.

Maureen Maher: Did you think anything about the car when you saw it, or the guy?

Rob Leef: I mean it was late. But no.

The next day, Leef flew to Las Vegas on a planned trip with old friends.

Rob Leef: Reading the news on my phone. … Saw a murder story. I clicked on it and I saw the address.

Maureen Maher: And right then do you think, "I got the guy?"

Rob Leef: I thought there was a high probability that I had something relevant and I needed to get back to Austin.

Leef raced back to Austin. He checked the video of that stranger on his street and called Austin police.

Maureen Maher: How important does the video end up being in Kathy Blair's case?

Det. Kerry Scanlon: Very -- very important.

The video was tantalizing, but blank on images that did not give off heat. So you couldn't actually identify the man, a license plate number or even if there was anyone else inside the vehicle. Still, there was one important clue.

Det. Kerry Scanlon: It gave us an idea what kind of vehicle our murder suspect arrived at the crime scene with. … And it was a sedan of some sort.

These cops needed much more evidence. Then, nine days after Kathy Blair's murder, in another peaceful Austin neighborhood, just 15 minutes from Kathy's home, they would get it.

Det. Derek Israel: The victims are an elderly couple … murdered overnight.

Maureen Maher: Viciously.

Det. Derek Israel: Viciously.

Sidney Junior, Johnny and Brenda want their parents to be remembered as the outstanding people they were, not the grim headlines they became. Billie and Sidney Shelton were hard-working, devoted to family, and happily married for 64 years.

Sidney and Billie Shelton Sidney Shelton Jr.

Johnny Shelton: We were never rich, but … not once did daddy ever complain about it. Not once did mama ever complain about it.

A life well-lived, and peacefully slowing down. Billie was 83, her husband Sidney was 85.

Dow Kotrla: These are sweet people that'll, you know, send you on your way with some cookies.

Home nurse Dow Kotrla was making her scheduled visit on Dec. 15, 2014.

Dow Kotrla: I knocked … nobody answered. …The front door was splintered. It was clearly -- had been busted open.

Dow nervously headed to the Shelton's modest bedroom.

Dow Kotrla: Their room had been ransacked. … And then to the left I saw him …. on the bed … And I – I ran. I just turned around and I ran.

Sidney and Billy Shelton had been beaten and stabbed.

Maureen Maher: The knife is still present?

Det. Derek Israel: Yes.

Maureen Maher: In one of the victims?

Det. Derek Israel: That's correct.

Maureen Maher: Is it clear that it's also a burglary?

Det. Derek Israel: Yeah, I was seeing some of the same things.

The "same things" found at Kathy Blair's murder scene. Starting with an empty jewelry box.

Det. Derek Israel: And again, the drawers are pulled out, they had been emptied and stacked.

"I thought we were dealing with a serial killer," said Det. Kerry Scanlon, left. "If word gets out that it's a serial killer, it kicks it to a whole other level," said Det. Derek Israel. CBS News

Three people had been slaughtered in their own beds. The crime scenes were eerily similar. And investigators were privately wondering, was there a serial killer loose on the streets of Austin.

Det. Derek Israel: If word gets out that there is a serial killer, it kicks it to an entirely different level.

So, investigators kept their worst fears to themselves. But why would any killer target Kathy or Billie and Sidney, who cops determined didn't even know each other?

Det. Derek Israel: None of these people had any enemies that we could figure out.

Det. Derek Israel: What is it that connects these people together? Besides the killer.

Maureen Maher: There's nothing for either Kathy Blair or the Sheltons that stands out to either one of you?

Det. Derek Israel: Nothing.

Det. Kerry Scanlon: Nothing.

Every lead was chased down. Then, almost three weeks after Kathy was killed, the name of a stranger surfaced: Tim Parlin. He'd done yard work at Kathy's house. And a friend reported Parlin was weird and rude.

Det. Kerry Scanlon: I go to our computer system.

It was a simple and easy search. Tim Parlin had spent decades in prison.

Det. Derek Israel: And he stole jewelry.

Det. Kerry Scanlon: And he stole jewelry.

Maureen Maher: Specifically, jewelry.

Det. Kerry Scanlon [affirms]: Mm-hmm. At night.

Maureen Maher: Are you hopeful at this point?

Det. Kerry Scanlon: I am.

Det. Derek Israel: This is the InTown Suites … Tim Parlin … where he was living at the time that the murders happened.

Israel and Scanlon went to look up the life-long convict and snapped photos of Tim Parlin.

Detectives photographed Tim Parlin outside of the hotel where he was living at the time of the Austin murders in 2014 Court evidence

Det. Derek Israel: Told him, we're homicide detectives

Det. Kerry Scanlon: So, he asked us a few questions as well.

Maureen Maher: About the murder?

Det. Kerry Scanlon: Well, yeah, like so, "How'd she die?" You know, stuff like that.

Maureen Maher: Wow that's bold … He's sussing out to see …

Det. Kerry Scanlon: What we know.

Maureen Maher: Cat and mouse.

Det. Derek Israel: It is.

Parlin spoke to the cops in the hotel's parking lot. But when they asked to see his room he refused, claiming his wife was inside and asleep.

Maureen Maher: And you drive away and what's the conversation?

Det. Kerry Scanlon: I said, "This is our guy".

Maureen Maher: You did.

Det. Kerry Scanlon: Yeah. And Derek says, "Well, I don't know yet."

A return trip to the InTown Suites, just a few days later pays off. Parlin wasn't home, but his wife was.

Det. Derek Israel: Explained what we were investigating. And she knew the Sheltons. His wife knew the Sheltons.

Tim Parlin's wife knew the Sheltons from church. And Tim Parlin had worked in Kathy Blair's yard. It was tenuous, but it was a connection.

Det. Derek Israel She gave us permission to search the apartment.

Maureen Maher: Did you find anything?

Det. Derek Israel: We did. A pawn receipt … for a piece of jewelry, a nugget pendant. … And it turned out that pendant belonged to Kathy Blair. … It was pawned on the same night that Kathy Blair was murdered.

But one thing didn't add up. Tim Parlin drove a van. And in that grainy nighttime video, shot by Rob Leef the night Kathy Blair was killed, Israel said, "It's definitely not a van. It's a car."

Det. Kerry Scanlon: So, we found out that his sister had a green Toyota.

A side-by-side view of the car seen in the grainy thermal scope video and Tim Parlin's sister's green Toyota Court evidence

Parlin had been using his sister's car, a green Toyota. Its outline appeared similar to the car in Rob Leef's video and one caught on security footage approaching that Austin pawn shop less than 24 hours after Kathy was murdered.

Det. Kerry Scanlon: We took it. We had it towed.

The car was towed and tested. On the passenger seat was a brown stain -- traces of dried blood.

Det. Derek Israel: The blood belonged to Kathy Blair that was in that car.

Austin was on edge and hoping for an arrest.

CBS AUSTIN REPORT | HEMA MULLUR: Kathy Blair was found dead inside her home … The wait for justice has been troubling for her students, family, and friends.

Hema Mullur | CBS Austin anchor : It was very very shocking in the community … and it was really unsettling.

But now, justice was closing in on one man: Tim Parlin.

Maureen Maher: But you're thinkin' one guy still?

Det. Kerry Scanlon: Oh, yeah.

A MYSTERY MAN

When Kathy Blair's blood was found in Tim Parlin's car, detectives Israel and Scanlon were convinced that he had killed her.

Det. Derek Israel: At that point we're all jubilant, we're super excited. We got our guy.

Parlin fit the bill perfectly. He had done yard work for Kathy Blair and was a career criminal with a long rap sheet of burglaries.

Det. Derek Israel: Now I just need to question him, confront him, hopefully he'll confess. But if he doesn't, we have hard, physical evidence to tie this guy to the murder.

Tim Parlin Travis County Sheriff's Office

Israel had Parlin arrested for an unrelated parole violation and brought in for questioning.

Det. Derek Israel: It seemed like a pretty short and straight road to charging Tim Parlin with murder. It turned out it wasn't a short road, and it certainly wasn't a straight road."

The first step was to get Parlin to corroborate some of details of Kathy's murder.

Det. Derek Israel: I just straight up told him that...you know, we knew that he had killed uh, Kathy Blair."

Maureen Maher: What's his response?

Det. Derek Israel: "I didn't do it." And … this was the thing he really liked to say: "These hands didn't kill anyone."

So, the detectives asked him, "Who did?" But Parlin wasn't giving up that information so easily.

Det. Derek Israel: So, after hours of this conversation he finally says, "OK, I'll tell you who it was." And that's when he said, "Shawn Gant-Benalcazar."

Maureen Maher: Who is that?

Det. Derek Israel: That's what I said. "Who is that?"

Maureen Maher: Did you think he was stalling?

Det. Derek Israel: Oh yeah, yeah, yeah … This sounded … completely made up. … And I knew as soon as he said it … that he had screwed us.

Maureen Maher: Because now, you had to go chase it down.

Det. Derek Israel: Because now we gotta go hunt some mystery guy down and prove that he didn't commit a murder.

With their lead suspect behind bars for now, the detectives reluctantly contacted Parlin's mystery man.

Shawn Gant-Benalcazar had never been in trouble with the law. He had a degree in microbiology, was once a high school science teacher and seemed an unlikely acquaintance of a serial jewel thief.



Det. Derek Israel: This guy lived in Galveston, he didn't cause any trouble.

Gant-Benalcazar readily agreed to meet with them that night at the Galveston Police Department:

DET. DEREK ISRAEL: Have a seat on the couch there. SHAWN GANT-BENALCAZAR: Alright. DET. DEREK ISRAEL: We're investigating a murder in Austin and, in particular, we're looking at Tim Parlin as the person that we -- we believe committed the murder. SHAWN GANT-BENALCAZAR: I'm completely in the dark on this, um. Who was murdered?

He tells the detectives he barely knows Parlin, and that he just met him a few months earlier when his sister began dating Parlin's nephew:

Det. Derek Israel: So, we started talking. "OK, when's the last time you were Austin?"… "Well, I've been in Austin a few times over the last month." … He was in town … during the weekends of both of those murders. And on top of it, he was staying with Tim Parlin.

SHAWN GANT-BENALCAZAR [interrogation]: The thing is, honestly, I wanna help you guys out because this guy ... wolf in sheep's clothing, basically. He didn't tell me anything about his -- his past, and I'm starting to feel like he set me up like a patsy or something.

The detectives knew from Parlin's criminal record that he did have a history of being a master manipulator.

DET. DEREK ISRAEL [interrogation]: Did Tim ever approach you about doing burglaries?

Before long, the mystery man who initially said he knew nothing slowly started to crack. Gant-Benalcazar now says he was sitting in the car when Parlin went into Kathy Blair's house:

DET. DEREK ISRAEL: So where are you sitting in the car? SHAWN GANT-BENALCAZAR: Passenger seat.

Det. Derek Israel: Well, if he's sitting in the passenger seat, then why is there blood in the passenger seat?

But Gant-Benalcazar had an explanation. He says Parlin came back to the car clutching a bloody pillowcase:

SHAWN GANT-BENALCAZAR: Came back with this sack. Had blood on it. Threw it on the passenger floorboard. And I took a peek at it. It had jewelry in it and I didn't want anything to do with it. DET. DEREK ISRAEL: When did you figure out it may've had something to do with a murder? SHAWN GANT-BENALCAZAR: The fact that it had blood on it was not a good sign.

Then, about four hours into the interview:

SHAWN GANT-BENALCAZAR: Just wondering, Can I use the restroom?

As he walked down a hallway …

Det. Kerry Scanlon: And he was walking in front of me, I looked up and it just, I mean, I got a chill, because I was like, "that's the same walk as the guy in the video."

Remember that spooky thermal video that the cops think accidentally caught Kathy Blair's killer ambling down the sidewalk?

Det. Kerry Scanlon: You know it's just the broadness, the deliberate steps … I thought it was him, I definitely believed it could be him.

Was it actually Gant-Benalcazar -- and not Parlin -- who had gone into Kathy Blair's house?

Det. Derek Israel: We started pursuing, you know the line of questioning ... along the lines of maybe he was in the house.

DET. KERRY SCANLON: Did he bully you into going into the house or --? SHAWN GANT-BENALCAZAR: I was scared and y' know, he -- he was taking a threatening tone … He told me to go in the house and get the stuff.

Det. Derek Israel: And finally he admitted, "I did go into the house. And I did steal the jewelry…"

DET. DEREK ISRAEL: You came in through where? The backdoor you said? SHAWN GANT-BENALCAZAR: Yeah. It was open. DET. DEREK ISRAEL: OK. Open, unlocked? SHAWN GANT-BENALCAZAR: Unlocked, yeah … I looked around and kind of prowled and snuck through quietly. … I turned on a couple of lights in rooms where I didn't see her. I found the room where she was, and she was fast asleep. That was the room her jewelry box was in. And so, I -- I opened the jewelry box, took the stuff out … put it in the -- the thing … Maybe he went back, I don't know, but I didn't kill her.

Det. Derek Israel: I told him, "That's impossible. Everything you said is true, except that it's not possible that she was still alive when you left …" And I explained, "That the person who turned on those light switches, you talked about turning on. The person who removed that pillowcase, you talked about removing. The person who removed those drawers, you talked about removing. That person had Kathy Blair's blood on his hands. So, the person who did all that, killed Kathy Blair." … " kept pushing him for the reason … "Something happened in that room when you were there. What happened?" And that's when he said, "I was standing there, I was looking at her…"

Motioning with his hands, Shawn Gant-Benalcazar describes to detectives how he killed Kathy Blair. Court evidence

With no room left to lie, he breaks down:

SHAWN GANT-BENALCAZAR: [sighs] She woke up, she lunged at me. Grabbed the knife, started trying to wrestle it out of my hand. And then -- it was a struggle [raises and lowers his clasped hands in a stabbing motion] and I stabbed her in the neck.

The confession came unexpectedly. The witness was now the prime suspect.

Det. Derek Israel: We had gone to clear the guy, and instead he confessed to capital murder.

Gant-Benalcazar kept talking and claimed that after murdering Kathy Blair, he handed off her jewelry to Parlin:

DET. DEREK ISRAEL: You didn't get to keep any of it? SHAWN GANT-BENALCAZAR: No, he didn't give me anything. I got nothing … You're gonna to have to ask him where he fenced it.

Shawn Gant-Benalcazar appears to have gained absolutely nothing from this senseless murder.

Det. Derek Israel: I've never met anyone who would go into someone's house and sneak in at night and, and murder them in their bed.

Maureen Maher: For what reason?

Det. Derek Israel: None. For their own gratification. That's it.

DET. DEREK ISRAEL [interrogation]: We're going to place you under arrest for capital murder..."

Detectives immediately read Gant-Benalcazar his rights and wanted to learn what he knew about the murder of the Sheltons.

Shawn Gant-Benalcazar Travis County Sheriff's Office

Det. Derek Israel: We started talking again. … Asked him about the Sheltons.

Try as they might, Gant-Benalcazar wasn't talking anymore.

Det. Derek Israel: "Well I wasn't there for that one, I don't know anything about that one."

Det. Derek Israel: By this point, everyone is exhausted. And, so eventually he just, he terminated the interview.

Maureen Maher: He said, "I'm done."

Det. Derek Israel: "Done."

SHAWN GANT-BENALCAZAR [interrogation]: Wish this all had never happened.

After the arrest, Detective Scanlon made a video of Gant-Benalcazar on his cell phone. His hunch seemed right.

Maureen Maher: That's the moment that you think, "It's him."

Det. Kerry Scanlon: Yeah. That's when I thought it was him.

Four days later, Austin police announce that they had made two arrests.

CBS AUSTIN REPORT | MALE ANCHOR: Two men are in jail in connection with the murder of a beloved choir teacher. … 30-year-old Shawn Gant-Benalcazar of Galveston is charged with capital murder. CBS AUSTIN REPORT | HEMA MULLUR: 49-year-old Timothy Parlin is ... also expected to face charges related to Blair's murder.

Hema Mullur: Shawn Gant-Benalcazar -- he was a UT graduate, he had no criminal record of any kind – how did he get involved with a crime like this?

Tim Parlin had an answer. Later, while in custody, Parlin admitted to the cops that he had driven Gant-Benalcazar to Kathy Blair's house and to the Shelton residence on the nights they were murdered.

And Parlin says he knew all along that Gant-Benalcazar had killed all three of them.

Investigators now thought they understood what happened.

Det. Derek Israel: Shawn murdered the Sheltons. Tim Parlin was a party to that murder. He planned it, he facilitated it, he profited from it. He assisted in it.

Maureen Maher: But you are 100 percent convinced that it was Shawn who murdered that couple.

Det. Derek Israel: Yes.

Maureen Maher: Will he ever be brought to trial for it?

Det. Derek Israel: It seems unlikely.

Unlikely, because there was no direct evidence linking Gant-Benalcazar to the Shelton murders – he would always deny he killed them.

Prosecutors would focus instead on building their strongest case, using Gant-Benalcazar's confession to convict him of killing Kathy Blair.

But when Shawn Gant-Benalcazar finally gets his day in court, no one could have anticipated what happened next.

Prosecutor Andrea Austin: It makes you worry because … this guy cannot be out on the streets.

SEEKING JUSTICE

Three years after Shawn Gant-Benalcazar confessed to the murder of Kathy Blair, his trial begins.

Prosecutor Andrea Austin: You never know what a jury's going to do, but, it was a very, very strong case.

Assistant D.A.'s Andrea Austin and David Levingston present the state's case:

PROSECUTOR ANDREA AUSTIN: The man who sits among us in this courtroom, the defendant Shawn Gant-Benalcazar, is Kathy Blair's killer.

Maureen Maher: How hard was it to be there during that trial?

Kirsten Mathiesen | Kathy Blair's sister: It was really hard. There were some pictures throughout that that I saw that I can't unsee.

Kathy's neighbor, Rob Leef, testifies about the video he recorded on the night she was murdered:

ROB LEEF: There was a car that – that parked … while I was out on the walk.

Rob Leef [to Maher]: And by the time I zoomed and zoomed back in, someone was already out of the car and was crossing onto the sidewalk.

"This video showed the murderer walking from the car towards Kathy Blair's house," Det.Derek Israel told Maureen Maher. "The actual killer." Court evidence

That someone, the prosecutor tells the jury, was Shawn Gant-Benalcazar on his way to murder Kathy Blair.

SHAWN GANT-BENALCAZAR [interrogation]: I just kinda looked through the rooms. You know, and I – I turned on a couple lights in rooms where I didn't see her.

The prosecution's case hinges on Gant-Benalcazar's rambling five-hour confession where he describes breaking into Kathy's house:

SHAWN GANT-BENALCAZAR [interrogation]: [Sighs] She woke up. She lunged at me. PROSECUTOR ANDREA AUSTIN: He had a knife out ... they fought over the knife … SHAWN GANT-BENALCAZAR [interrogation]: And I… stabbed her in the neck.

Det. Derek Israel: He didn't just kind of confess. … he straight up confessed to all the details of – of killing Kathy Blair.

That confession was vital to the prosecution's case.

Prosecutor David Levingston: He gave enough details in this confession that were kept out of the media … so we could show the … confession was from the actual killer, and that he knew enough about this crime to have either been there or done it himself.

DEFENSE ATTORNEY ARIEL PAYAN: There's no question that this is a horrible, horrible crime.

But Gant-Benalcazar's defense lawyer Ariel Payan makes a bold accusation right off the top -- that damning video? It was all a lie – a false confession coerced by Detectives Israel and Scanlon.

The defense tells the jury that during that 5-minute bathroom break in the hallway when Gant-Benalcazar was not being recorded, detectives threatened him:

DEFENSE ATTORNEY ARIEL PAYAN: Law enforcement went down there, we believe the evidence will show, with the express idea, plan, purpose, and intent, to try to get him to confess to something he didn't do.

Prosecutor Andrea Austin: They have to come up with something … they have to argue that it's an involuntary statement, but we obviously knew that wasn't true.

During his trial for the murder of Kathy Blair, Shawn Gant-Benalcazar takes the stand and blames investigators for his confession.

Gant-Benalcazar takes the stand to blame the cops for his confession:

SHAWN GANT-BENALCAZAR [testifying]: Their exact words were, "This is important and we're not – you're not going anywhere until we finish." SHAWN GANT-BENALCAZAR [testifying]: That if I – I didn't explain a reason for having done it -- even though I didn't do it – um, I would get the death penalty.

And he maintains that it was actually Tim Parlin who killed Kathy Blair on that chilly December evening back in 2014.

Maureen Maher: Were you worried the jury might believe it?

Det. Derek Israel: Well, you always have to worry with juries.

PROSECUTOR DAVID LEVINGSTON: You don't get blood on your hands and put it on a jewelry chest …

At closing arguments, prosecutors insist Gant-Benalcazar voluntarily confessed, and offered details about the crime only the killer could have known.

Prosecutor David Levingston: I think it comes...down to … a credibility --

Prosecutor Andrea Austin: And hopefully, you're sitting there thinking, "This guy confessed, why are we here?"

The case goes to the jury.

Maureen Maher: When the hours started ticking away with deliberations, two, three, five … six, seven, eight, nine…

Prosecutor Andrea Austin: You feel – you feel awful.

Maureen Maher: Were you worried?

Det. Derek Israel: Yes. The idea that he would get out -- it was just unthinkable. I mean, Shawn is gonna kill somebody else if he got out.

After 19 hours of jury deliberations, the judge declares a mistrial. The jury cannot reach a verdict.

Det. Kerry Scanlon: One person held out. … She didn't want to consider the confession.

Prosecutor Andrea Austin: I mean, look, that's what the system is about. We -- we're required to get a unanimous verdict, we didn't get a unanimous verdict.

Maureen Maher: How hard was it to hear that there was a mistrial and you would have to go through it all over again?

Kirsten Mathieson: Really hard. Yeah, that was – that was tough.

With a retrial in the works and Tim Parlin's trial less than a month away, prosecutors were worried. Could they get any jury to convict either of these men?

JUSTICE SERVED

PROSECUTOR ANDREA AUSTIN: This man, Timothy Parlin … knew that Shawn Gant would go in and murder Kathy Blair.

With Shawn Gant-Benalcazar's mistrial still fresh in her mind, Prosecutor Andrea Austin is determined to put Tim Parlin away for life.

He stands trial for both the murders of Kathy Blair and the Sheltons.

Prosecutor David Levingston: In Texas, if you were part of the crime, then you are also guilty of that crime … You can convict him, even if you don't believe he stepped foot inside that house.

Maureen Maher: Because he was there and he participated.

Prosecutor Andrea Austin: Correct.

PROSECUTOR ANDREA AUSTIN [addressing jurors]: I'm going to ask you to ... find him guilty of capital murder.

Detectives were convinced Gant-Benalcazar had killed the Sheltons, but had no evidence to charge him with their murders.

Tim Parlin stood trial for the murders of Kathy Blair and the Sheltons. CBS News

So, Tim Parlin would prove to be an easier target for prosecutors.

Parlin admitted he drove Gant-Benalcazar to both murder scenes and the car Parlin was driving had Kathy Blair's blood in it.

Kirsten Mathieson: Oh, he did much more than sit in the car. He's the one who targeted Kathy. He's the one, for whatever reason said, hey, you know what this would be a good person for you to murder.

Parlin's lawyer, Keith Lauerman, argues that despite his client having confessed to driving Gant-Benalcazar to both murders, there is no evidence placing Parlin inside the two the houses.

DEFENSE ATTORNEY KEITH LAUERMAN [to jurors]: He never set foot in either one of these homes … And at the very end, you're gonna realize that this man and those hands never participated in any murders.

After a nine-day trial, the jury didn't take long to reach a verdict: guilty of capital murder.

Parlin is sentenced to mandatory life in prison, without the possibility of parole for his role in the murders of Kathy Blair and Sidney and Billie Shelton.

Five months later Gant-Benalcazar went to trial a second time -- for the murder of Kathy Blair.

Prosecutor David Levingston: The nerves were much higher the second round.

Maureen Maher: Well, there's a lot at stake.

Prosecutor Andrea Austin: There's a lot at stake

And again, Rob Leef's testimony is critical for the prosecution:

PROSECUTOR DAVID LEVINGSTON: At some point did you see an individual get out of that car? ROB LEEF: Yes sir, I did. PROSECUTOR ANDREA AUSTIN [addressing jurors]: I want you to watch this. SHAWN GANT-BENALCAZAR [interrogation]: It was a struggle and I ... stabbed her in the neck. PROSECUTOR ANDREA AUSTIN: Look what he does with his hands. He's retrieving a memory, right? Involuntarily, he's doing this ... He remembers doing, because he's the one who murdered her.

Once more, Gant-Benalcazar swears the cops coerced his confession:

SHAWN GANT-BENALCAZAR [testifying]: The police wouldn't let me go. SHAWN GANT-BENALCAZAR [testifying]: And I come out of the bathroom and they keep saying, "Oh, we know you did it. There's no doubt you did it" and they keep saying it, keep saying it, and I just got worn down.

This time out the jury deliberates less than three hours.

JUDGE: We the jury find the defendant, Shawn Gant Benalcazar, guilty of the offense of capital murder.

Prosecutor David Levingston: We poured our emotions out into this case.

Prosecutor Andrea Austin [emotional] : It was justice delayed, and yeah ...

Maureen Maher: But delivered.

Prosecutor Andrea Austin: But delivered.

Like Tim Parlin before him, Shawn Gant-Benalcazar was sentenced to life in prison without the possibility of parole.

KIRSTEN MATHIESON [POST-TRIAL PRESS CONFERENCE]: We're pleased. We miss Kathy. This isn't gonna bring her back. Our hearts are never gonna completely heal.

A few months after the verdict, Maher spoke with Gant-Benalcazar via a prison video phone. She wanted to ask why? Why would a guy who'd never been in trouble with the law suddenly turn into a vicious killer?

Instead, with no evidence of any remorse, he repeated what he'd told the jury: that he was innocent, the cops had forced him to confess, and that Tim Parlin was the one who had killed Kathy Blair:

Shawn Gant-Benalcazar: When I went to go see him at four, four a.m. … um, he said "Well, we're gonna go get breakfast." And, uh, drove me out to the place, and then said that he had killed her, and told me about it.

Maureen Maher: Tim Parlin confessed to you, that he killed Kathy Blair?

Shawn Gant-Benalcazar: Yeah, that's right.

Maureen Maher: How, how could you have known the movements of the killer?

Shawn Gant-Benalcazar: Anything that, um, I said was something that either uh, y'know, Tim told me or I just made up.

And not surprisingly, when Maher visited Parlin at a prison in northeast Texas, he pointed the finger at Gant-Benalcazar, and claimed he knew absolutely nothing about the murder of Kathy Blair:

Tim Parlin tells Maureen Maher that Shawn Gant-Benalcazar is responsible for the murder of Kathy Blair. CBS News

Maureen Maher: After Shawn viciously kills Kathy Blair, gets back in your car and drives away, and he goes back to Galveston.

Tim Parlin: He never said a thing.

Maureen Maher: Never said a thing.

Tim Parlin: He never said a thing. A stone-cold individual actually.

Maureen Maher: You've been described as the master manipulator.

Tim Parlin: [laughs]

Maureen Maher: That you talked him into doing it.

Tim Parlin: My IQ is very low --

Maureen Maher: Oh, I don't believe that.

Tim Parlin: -- and I have a big heart.

Maureen Maher: I don't believe that.

Tim Parlin: It is, it's very low actually. And I have a big heart, so I'm not the mastermind behind anything.

Maureen Maher: You're just a big teddy bear behind bars.

Tim Parlin: Yeah, pretty much.

Rob Leef is the accidental hero of this story – someone who never knew he'd be called upon to help solve a murder.

Austin citizen Rob Leef is commended for his role in helping solve a murder and making Austin a safer place. pic.twitter.com/TLDIHFELtx — Austin Police Dept (@Austin_Police) April 22, 2015

Maureen Maher: And you ended up leaving the neighborhood?

Rob Leef: I did.

Maureen Maher: Needed a change?

Rob Leef: I did, absolutely.

Kristin DeGroot: I would not be where I am as a – a professional actor and musician without her influence.

Kathy Blair's student, Kristin DeGroot, is moving to New York to pursue her dream of a career in music.

Kristin DeGroot: One of my greatest regrets is that I never was able to tell her that she did this for me.

Maureen Maher: How proud do you think she would be of you?

Kristin DeGroot [cries]: I hope she'd be really proud of me.

Det. Derek Israel: They're just evil people ... in the end it's just two -- just broken human beings who, you know, basically put a path of destruction through, you know, two families.

Kathy Blair, left, and Sidney and Billie Shelton Kirsten Mathieson/Sidney Shelton Jr.

Two families who will forever share the same tragedy.

Johnny Shelton: They were the best parents you could ever want.

Kirsten Mathieson: I just miss her … And at the end of the day, she's gone. And I can't call her tonight.

Learn more about the scholarship created by Kathy Blair's family and the Texas Choral Director's Association in her memory.