An aspiring small town model with big city dreams, her beautiful face would soon be on the screen. But not as a superstar -- as a murder victim.

Now her killer is on the loose and her family lives in fear. Police zero in on security footage and a mystery man who could hold the key to cracking the case. Who is he?

Shelbey Thornburgh didn't have much of a childhood. She was allegedly abused and neglected, shuttled from one foster home to the next through rural central Texas.

Shelbey's sister Krystina Scott says early on, depression set in, leading Shelbey to the brink of obesity.

"She used to be very overweight and she had determination to make herself where she wanted to be," said Krystina.

Through it all, Shelbey had her loving guardian angel, her big sister Krystina. After they grew up, Krystina helped her little sister with adult dreams, moving to the big city: Houston, Texas. And Krystina helped Shelbey achieve her goal weight. It all paid off.

Soon, Shelbey had a boyfriend, an apartment in a luxury high-rise, and big ambitions.

"She had told me she wanted to become a model," said Krystina.

Finally, Krystina could relax, knowing that Shelbey's life was on track.

But life in the big city wasn't as rosy as it seemed. Under the bright lights, Shelbey's dreams of modeling soon came with a high price tag.

"She was almost 300 pounds at one time," said Krystina. "And it left her with some flab, so she wanted to get the tummy-tuck done and get a little bit bigger boobs and so that way she could have no problem becoming a model."

Desperate for cash, Krystina claims Shelbey turned to a job with the encouragement and help of her own boyfriend, a job her sister disapproved of: A high-end escort.

"She was using the escorting business as a way to get her stuff done that she wanted to become a model," said Krystina.

Every time the phone rang, Krystina feared it was Shelbey calling from jail. And one day a call did come through, this time with far more tragic news.

"She said 'Are you Shelbey Thornburg's sister?' And I said 'Yes, ma'am, I am,' and she said 'I'm sorry to inform you that your sister has passed away,'" said Krystina. "I get woke up at 4 in the morning to a phone call telling me my sister had passed away."

Krystina soon learns Shelbey was brutally murdered on Nov. 4, 2015. Even more shocking are the grisly details.

"It appeared to be a pretty vicious attack," said Houston Police Homicide Detective Dustin Crowder. "Her throat was slashed from left to right with a very sharp knife, so sharp that it actually cut into her bone of her neck."

"They told me she was cut on the neck and had cut on her wrist like she put up for self-defense," said Krystina.

Who would want Shelbey dead? And why?

"The 911 call came in when Shelbey's boyfriend called the police, he came into her apartment and found her lying on the, bed unresponsive," said Det. Crowder.

Krystina went to Houston, determined to help detectives crack the case.

"The first impression I got when my partner and I arrived, Shelbey was nude, so obviously this person had some sort of relationship with her," said Houston Police Homicide Detective Dustin Crowder. "There's no evidence right now that indicates that she was sexually assaulted. It appears to have been consensual."

Consensual -- until it turned deadly.

Detectives tell Crime Watch Daily that Shelbey's boyfriend has a lengthy drug-related rap sheet, although no convictions for violent crimes. He comes in for questioning three times, always maintaining his innocence.

"We investigated the boyfriend extensively, and most certainly we haven't ruled him out as a suspect," said Crowder. But police have no actual evidence against the boyfriend to make an arrest for murder.

Months pass. A grieving sister is left wondering if Shelbey's killer will ever be found.

"It's really important to me to find out who killed Shelbey, not only for Shelbey, but so that way he don't do it again," said Krystina.

But then, a bombshell from Shelbey's own apartment building.

"The high-rise apartment complex had a video system in the lobby only, and we did obtain that video, and the suspect is seen entering and leaving the apartment complex," said Crowder.

Four months after the murder, police release stunning surveillance footage showing a suspect entering Shelbey's building just moments before investigators believe she was killed. The man on the tape is clearly not Shelbey's boyfriend. Police describe him as a white male wearing a light-colored, long-sleeved shirt, shorts and tennis shoes.

The time stamp on the footage reveals the mystery man entered Shelbey's building at 8:34 p.m. He left just 23 minutes later, at 8:57, right around the time police believe Shelbey was murdered.

"It seems as if the suspect committed the crime and then made sure to be gone as quickly as possible," said Crowder.

Given the time frame of her death, cops believe he could be the killer, and they are desperate to know if he was connected to Shelbey, and did he want her dead?

Another clue comes from Shelbey's own phone, a message exchange with a man setting a date, most likely a reply to her escort ad.

"We are confident that person that's seen in the video was going to be the suspect and not the boyfriend of the victim," said Crowder.

If police can trace the man in the video to the phone number, they might possibly make a connection.

"The phone was purchased shortly before this incident and then deactivated immediately afterwards, so the phone that the suspect used was in fact just for this purpose and then deactivated and no longer traceable," said Crowder.

A "burner" phone, paid for in cash so it couldn't be traced. And it leads Detective Crowder to a chilling potential new theory: Shelbey could be the victim of a serial killer.

"I would say that the suspect in this case appears to have made contact to our victim based on her appearance," said Crowder. "Shelbey was a blonde, an attractive woman, and he may have had a preference for that type of woman."

Police tell Crime Watch Daily the suspect was contacting other women similar in appearance before the night of the murder. He was careful not to leave fingerprints, and clearly knew what he was doing. They are now coordinating on their theory with the FBI.

"He had to have some knowledge on how deep to cut the throat, where to cut the throat, what to wear, how to do it," said Krystina. "He walked out not even with a spot of blood on him. He walked out with the same clothes he walked in with."

Detectives broadcast the video throughout Houston and posted a reward.

Meanwhile, Shelbey's sister wages her own campaign online, desperate to find the killer before he strikes again.

"We do have a GoFundMe page going, we want to hire a private investigator to work solely on Shelbey's case," said Krystina. "Our goal is to raise $20,000."

It's been nearly a year since Shelbey's murder, and so far, no leads have panned out.

"The case is ongoing and it is being investigated and we've actually received tips up until this day," said Det. Crowder.

In a Crime Watch Daily exclusive, we've brought in well-known video forensic analyst Doug Carner, who worked on the Trayvon Martin case, to enhance the surveillance footage, hoping to get a cleaner look at the possible killer. The quality of the original video only allows for slight improvement, but it's enough for Carner to reveal some surprising new information.

"From the original recording, it's very difficult to see for sure what's on his head," said Carner. "As we move over to the enhanced, we see what appear to be sunglasses on his head, a slight receding hairline, and what appear to be shorts."

Then, a particularly telling detail not well-noticed in the original.

"At the time when he's leaving the building, it appears his left hand is in his pocket," said Carner.

Our expert is also able to enhance what could be most helpful to police. Shockingly, the suspect walks right past potential witnesses sitting in the lobby, including a man who comes through the door.

"In the enhanced video, we can see that the three people sitting here are paying attention to something in their laps that appear to be cellphones," said Carner. "It might explain why they did not notice the gentleman leaving."

But now, with the enhanced footage, Crime Watch Daily wonders if any of those witnesses in the lobby, or anyone else, will recognize the man in this scene. Cops are desperate for more information.

"We have put the video out to Crime Stoppers of Houston, and if anybody that watching this does recognize this person, they can contact Crime Stoppers of Houston, and there is a reward for any information that leads to the arrest of this suspect," said Det. Crowder.

Contact Houston Crime Stoppers at (713) 222-TIPS.

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