A mother checks on her teenager before she leaves for work. Everything appears to be OK. That girl will never been seen again.

Hailey Dunn was just 13 years old when she was reported missing. She was last seen alive on December 27, 2010. But what happened in the hours before would be a troubling mystery -- one with several different stories.

Hailey Dunn, a bright kid with a winning smile, grew up in the small town of Colorado City, Texas. Today she is the subject of a murder mystery that defies resolution.

Billie Dunn says the last time she saw her daughter Hailey was on an early morning a few days after Christmas.

"I was on my way to work, so just peeking in her bedroom, seeing her in the bed," Billie tells Crime Watch Daily. "She was completely home alone while I was going to be at work for my 12-hour shift. Her father did live right across the street so I assumed she was going to get up and go over there."

But when Billie returns home later that night, Hailey isn't around. Billie says her live-in boyfriend Sean Adkins is also back from work and tells her that Hailey went to stay the night at a friend's house nearby.

But the next day, still no sign of Hailey.

"And that's when we found out the friend said 'Hailey never spent the night here and I never saw her last night,'" said Billie.

Her daughter had vanished. Billie desperately begins searching for Hailey at all her friends' homes and at her ex-husband Clint's place down the street. She says she has a hard time believing that Hailey would run away.

"Anybody could have been driving through this town and saw my daughter playing outside or walking to the store and picked her up," said Billie.

Billie contacted police and files a missing-person report. The town rallies around her.

Crime Watch Daily affiliate CBS7 News Anchor Tatum Guinn has covered the story from day one.

"We had a story every day, a search update on Hailey for a very long time," said Guinn. "Any time human remains were reported, we were always on high alert, saying 'Oh, this could be her.' That group the whole time said 'No, we're holding out hope she's going to be alive, we're going to find her.'"

In the missing-person investigation, police turn to those closest to Hailey: Mom Billie, and her live-in boyfriend Shawn Adkins, and Hailey's dad Clint Dunn, who lives down the block. All of three have almost daily interactions with her.

As the investigation deepens, Hailey Dunn's home life is revealed as anything but normal.

"She loved her family. So home life seemed to be very simple, but the further we started learning about her family, things seemed to be different behind closed doors," said Guinn. "We got word about a lot of alcohol, a lot of partying from her mother, and so you got the sense that maybe home life was a little harder than what they let on."

And Billie Dunn and Shawn Adkins are on police radar even before Hailey's disappearance.

"About 10 months before Hailey went missing there was a call to police, a domestic disturbance," said Guinn.

The couple's relationship is tense and combative, with both calling 911 on each other. No one was arrested, but cops have sure have been getting an earful from the couple.

But Billie says they soon patched things up and Shawn moved into the house.

"He made me believe that he's not that person, those things won't happen again," said Billie. "I believed and wanted to believe that he was the good guy that I usually saw. So I kind of brushed it off."

"He was a friend. He was never made up to be a father figure or stepfather, never," said Billie. "He was just there to be her friend."

After a few weeks and still no sign of Hailey, cops focus on Billie and Shawn. Both agree to take a polygraph test.

Friends, neighbors and police comb the fields around town for any sign of missing 13-year-old Hailey Dunn. But her mother Billie never joins the search party.

"I wouldn't partake in the looking in fields because for me that meant we're looking for a body," said Billie Dunn. "I couldn't bring myself to do that, so I did the fliers and the Facebook and the media."

And what about her live-in boyfriend, Shawn Adkins?

"He wasn't involved in foot searches at all," said Billie. "He really didn't even help me pass fliers and things. I think he just wanted to stay out of the spotlight."

Police are so concerned that they ask both Billie and Shawn to take polygraph tests. Billie does so willingly. But she says she was shocked when she got her results.

Billie took two polygraph tests. According to a police affidavit, she was under the influence of narcotics when she sat down for the first test. So days later she sits down again, and the outcome is a shocker.

"I was told by the DPS interviewer that gave it that deception was indicated," said Billie.

Neither Billlie Dunn nor police will say on exactly which questions she showed deception. But the results make cops dig deeper. And according to the affidavit, a search warrant uncovers an alarming trove of disturbing literature in the house. Police found Billie and Shawn had collected pages and pages of articles printed from online content, all of it profiling serial killers. Among those was material about serial killer George Banks, convicted of 13 murders, including five of his own young children.

"When asked Billie told police, 'Oh that's just a hobby for us, we enjoy studying about those things,'" said CBS7 News Anchor Tatum Guinn.

Studying serial killers and her daughter goes missing? We had to ask: Just a coincidence?

"Anything to make me look bad," said Billie. "I mean, I'm not the only true-crime fanatic, I'm not the only viewer you have."

With that, Crime Watch Daily went down the street from Billie's house to talk to Clint Dunn, her ex-husband, and get his take on her reading habits.

"They were doing a lot of research, a lot of research for some reason. And it's just awfully peculiar that they did all this research, and then Hailey is missing," said Clint Dunn.

Clint says he is disgusted by what is happening at Billie's house in the days following Hailey's disappearance.

Does he feel Billie may have had some type of involvement?

"Yes," said Clint. "Because of all the actions that she made within the first week. They threw a New Year's party four days after she was missing. They had beer and watched the ball drop on New Year's while I was standing on my back porch with binoculars watching my house, waiting for Hailey to come home. That's how I know that they partied. I was watching them carry the beer in."

Billie says it was only a small family support gathering on New Year's Eve.

"Do we even realize it's New Year's Eve? No, my daughter is missing," said Billie.

But Clint is having none of it.

"I don't see how you can just celebrate the new year when your daughter is missing," said Clint. "That should be the last thing on your mind."

Surprisingly, Clint says he did not join the search party to help find Hailey either. He wanted to stay close to town instead.

"I wasn't going on the searches," said Clint. "I didn't want to find my daughter out in the country. I was walking down alleys and streets looking in every Dumpster and underneath every rock."

And it begs the question many have been wanting to ask him about the day Hailey went missing. Someone in Clint's life told police Hailey had come to his house on the day she went missing. Did police question Clint Dunn about that?

"Yeah, but she didn't come by," said Clint. He says he was never a suspect.

And what about Billie's live-in boyfriend, Shawn Adkins?

"This is a strange individual. Especially after Hailey was missing," said Clint. "I've talked to a lot of his friends in high school, and they all told me how strange we was. Obsessed with serial killers."

Billie Dunn failed a lie-detector test about the disappearance of her daughter Hailey. Police are not revealing what they make of that. But then it was her boyfriend Shawn Adkins's turn, and when cops hook him up to the polygraph machine, everything changes.

"The first two times he failed, got up and walked out," said CBS7 News Anchor Tatum Guinn, who's covered the story.

On the third try, investigators say, he shows deception. But the test indicates that Adkins is telling the truth on two bombshell questions that could break this case wide open.

"They asked him pointedly, 'Where can we find Hailey?' He said somewhere in Scurry County," said Guinn. "They asked him, 'Who should we be looking at in this?' And he said 'You should be looking at us' -- meaning Shawn and Billie."

Scurry County is about 20 miles north of where Hailey was last seen.

Shawn Adkins is named a suspect in Hailey's disappearance.

And now Billie tells Crime Watch Daily that police tried to use her to get more out of him: They wanted her to confront Adkins.

"They made me go in there, so I did," said Billie. "I felt faint-ish after asking him 'Where's my baby? Where's my baby? You failed your test! Where's my baby?'"

But Billie is about to find herself on the wrong side of the law. At one point during the investigation, police show up at her house looking for Shawn Adkins. Billie answers the door and tells them he's not home. But police have a search warrant, and when they go in, they find Shawn Adkins in the house. They don't arrest Shawn, but they haul Billie in for lying about his whereabouts.

Her attorney, John Young, says it's all much ado about nothing.

"That was really a non-issue," said Young. "They wanted to as where Shawn was, she said 'He's not here,' she opened the door for them, it was a non-issue. We went in, we accepted complete responsibility for not being completely honest. It was really, I believe, more of an effort to place pressure on her than anything else."

Clint Dunn sees it differently.

"I just don't see how she can hinder the investigation of her own daughter's disappearance, and she did so several times," Clint said.

But now Billie says even she's got some serious doubts about her on-again/off-again boyfriend Shawn Adkins.

"Maybe he's pretending to be this nice guy and my friend and my comforter," said Billie.

According to a police affidavit, Shawn told people he had been fired from his job the day Hailey disappeared, and left work early.

"But when law enforcement talked to his boss, that person said 'No, Shawn is a great employee,'" said Billie's attorney John Young.

In fact, Shawn's boss told police that Shawn left on his own early that morning.

"'He came in at 6 this morning and went into the break room, bought a Dr. Pepper,' his supervisor went in, he just looked at him and left," said Young. "Left the premises at about 6:10."

"I later come to find out from police -- not even Shawn himself -- police let me know Shawn had quit his job that morning," said Billie Dunn.

Does Billie think Shawn came to her house that day since he quit his job that morning?

"I think he probably did. He first lied and said he didn't come back there that morning. When we got cellphone pings, he did," said Billie.

The police affidavit shows Hailey's brother saw him at the house later that same day. Police say 16-year-p;d David Dunn pounded on the front door at about 4 o'clock that afternoon. All the doors were locked, so he went in through a side window and caught Shawn Adkins standing in the hallway with "a deer in the headlights look."

"We were always kind of suspicious of him and his actions, and it turns out in the court documents his story was changing with police, even," said Tatum Guinn.

"He has no alibi for the day that she disappeared," said Clint Dunn.

Then the most chilling of all from the police affidavit: Hailey's uncle stated that he could not believe anyone could hurt a 13-year-old little girl. According to the affidavit, Shawn replies, 'Yeah ... It's like killing a deer."

Then, the uncle goes on to say, Shawn Adkins talked often about how he kills deer and cuts them up with a chainsaw.

"I think he's a suspect, I think it's plausible," said Billie Dunn. "He bothers me, and I hate him."

What was it for her personally that triggered the thought that said maybe he's responsible?

"I later learned that it wasn't just Mitchell County calling him a suspect, I later found out FBI was also, and that was a game-changer for me," said Billie.

Despite being the only named suspect, Shawn has never been charged in Hailey's disappearance.

And then in March 2013 a break in the case: A hiker comes across a skull and a femur bone at Lake J.B. Thomas, a remote area about 20 miles north of Hailey's home.

"Her remains were found over a fairly wide area. Primarily bones," said attorney John Young.

A single tooth is found with the remains. Police confirm the identity through dental records. It's all that's left of beautiful 13-year-old cheerleader Hailey Dunn. Her mom gets the call from police.

"I really can't even remember other than hollering 'No, no, no,' and I was sitting on the floor in the bathroom at one point screaming 'No' and crying," said Billie. "I couldn't stop crying."

"My brother called me and said that Hailey's body had been found," said Clint Dunn, Hailey's father. "I was relieved, in a way, that she wasn't suffering. I had hope that this was what they needed to convict somebody."

But for some reason, Hailey's remains have never been released to her family. And the possible cause of death is still known only to police.

"I want a place to go visit and remember my daughter," said Billie. "And I want to know her remains aren't sitting on a shelf in a lab. It's very important to get her back, it's very important. I just try to make sure that I tell her every day 'I love you and I am thinking of you today.'"

"Certainly she should be given the peace of mind of 'how my daughter died,'" said Young. "Was it blunt-force trauma, was it a gunshot, what was it? I believe that she should have the closure, and the finality to put her daughter to rest."

And the place where Hailey's remains are found? It's in Scurry County. Remember, that's right where Shawn Adkins said she could be found during his polygraph test.

"The cellphone pings were picked up from Shawn's phone right in this area on the day that she disappeared," said Clint, while visiting the scene where Hailey was found.

Crime Watch Daily reached out to the Texas Rangers, who are now handling the case. But they have no comment on the many lingering questions.

Crime Watch Daily tried to track down Shawn Adkins himself, unsuccessfully. A person at the property did not agree to answer questions.

The case now gets colder by the day.

"There are so many facts and so many things inside this story, how come nobody is being held accountable?" said Tatum Guinn.

But Billie promises that will change.

"I'm a mom with a murdered child, and nobody's been held responsible," said Billie Dunn. "And I can't die not knowing somebody's been held accountable. God will help that person accountable. But I want to see it and I've vowed to my daughter that I won't stop."

"I'm going to find your killer," said Billie. "It's my job to take care of that now. Hailey has no worries. Be in heaven and sing with the angels. It's my job to get this done. I'm the mom. I'm your voice."

Clint Dunn, Hailey's brokenhearted dad, says he holds out hope for something, anything, from police that will bring justice for his daughter.

The Texas Rangers have reportedly confirmed they still consider Shawn Adkins a suspect in the murder and disappearance of Hailey Dunn, telling local television stations that the case is still active, and that they continue to receive tips to this day.

Hailey Dunn's family has a Facebook page featuring information and updates on the case.

If you have information about the case, contact Texas Rangers at (512) 424-2160, or the FBI's Texas Office at (972) 559-5000.