UPDATE March 30, 2018 2:07 p.m. PT: Michael Johnson was acquitted in the murder of Phylicia Barnes Friday.

She's a small-town girl visiting her big-city half-sister over the Christmas holiday. But before the high school honor student can ring in the new year, Phylicia Barnes vanishes without a word to anyone.

Her disappearance dominates the headlines. And her family's desperate pleas to the public galvanize a nationwide search. Suddenly a mysterious sex tape surfaces. And a sickening secret is unveiled.

What happened to Phylicia Barnes?

Phylicia Barnes, 16, idolized her older half-sister Deena.

"I know there was an age difference, but she was like my best friend," said Deena Barnes.

"They had a wonderful bond," said Russell Barnes, the girls' father.

Russell Barnes said the half-sisters grew up in different households, so it was a special treat for the college-bound athlete to get to know Deena and make new friends in the bustling city of Baltimore.

Did Russell ever fear for Phylicia's safety?

"No, not once, not at all," said Russell.

It was three days after Christmas 2010. Phylicia had decided to sleep in rather than go with her sister to her job. They would meet up later at home. But those best-laid plans took a mysterious turn.

"I called her phone numerous times. Repeatedly, like hang up, call back, hang up, call back," said Deena. "I was panicking at the point where I couldn't find her. I had called everybody I could think of and no one had heard from her, and I couldn't reach her. That's immediate panic, it's not like Phylicia."

Hours trickled by. Phylicia's obsessive postings on social media mysteriously came to a dead stop.

"That didn't take long at all for me to call the police," said Deena. "It was like within an hour of me getting home."

There's not a whisper of her whereabouts.

Investigative reporter Joy Lepola from Crime Watch Daily Baltimore affiliate Fox45 says with each new clue, it became clear Phylicia didn't leave willingly. She didn't take any of her money, and it seemed like she didn't take what she needed to be out anywhere in the cold.

"I believe she was in her slippers," said Lepola.

The search was looking grim.

Phylicia had a passing schoolgirl crush on Deena's ex-boyfriend, 28-year-old Michael Johnson, and he often volunteered as her chaperone.

"So I am sure there was some appeal. Whether she wanted to be involved in a relationship beyond friendship, that's hard to say," said Lepola.

But on the night Phylicia mysteriously disappeared, Michael Johnson reportedly called in sick to work and vanished for a while too.

"I called and asked him to come around. I couldn't find him," said Deena.

When Deena finally did reach Johnson she told him Phylicia was missing, and she was terrified.

"He was like, 'Well, do you want me to come around there, do you want me to come over? OK, keep me updated, keep me informed,'" said Deena.

Strange behavior from a man who playfully called Phylicia "Little Sis."

"If it was his brother or sister, I would have been out there in my car driving around looking. I would have come right away and I would have been looking," said Deena Barnes.

At some point Johnson did come over.

"I remember him saying 'This isn't good, I was the last person that saw her,'" said Deena. "I remember him saying that."

Not only that, Deena's ex had been moving his things out of her apartment on the same day Phylicia mysteriously went missing. And Johnson was seen dragging a 35-gallon plastic tub out of Deena's place and up the stairs into his new apartment.

"A neighbor came out across the way in the building and saw him struggling, trying to kind of push this plastic tote up the steps, and the neighbor asked him if he needed any help and he said 'No, I got it.' But he was struggling," said reporter Joy Lepola.

The disappearance of 16-year-old Phylicia Barnes sets off a firestorm in the media. The beautiful honor student went missing just three days after Christmas.

"I wouldn't think that she would just run off or anything like that, 'cause that was not in her character," said Phylicia's father, Russell Barnes.

But after an exhaustive four-month search, a father's worst nightmare: His beloved daughter's body is found, naked, floating near a dam in the Susquehanna River 45 miles from home.

Phylicia's dad took us to the spot where they found his Phylicia. He carries purple balloons and a bouquet of flowers to place at the water's edge of Maryland's Susquehanna River. He'd never been there before this visit. It's a courageous journey for a broken-hearted father.

Russell Barnes is now on a mission to make the person who killed his daughter pay. And Russell claims he knows right where to look: Michael Johnson, his older daughter Deena's ex-boyfriend. He was the guy seen struggling to move a 35-gallon tub shortly after Phylicia disappeared. A neighbor claims he offered to help, but Johnson told him to stay away.

"The theory goes that Phylicia's body was in that box," said reporter Joy Lepola.

Michael Johnson used to refer to Phylicia as his "little sis." But it turns out the 28-year-old was more than an unofficial friendly chaperone to the vulnerable teenager. Phone records show they had exchanged more than 1,300 texts over the last few months of her life.

"I am sure there was some appeal," said Lepola. "Whether she wanted to be involved in a relationship beyond friendship, that's hard to say."

And police say Johnson was the last known person to see the 16-year-old alive.

"He never disclosed his whereabouts. He was a person of interest from the beginning," said Russell.

Then a paper trail surfaces of Johnson buying that 35-gallon plastic tub. With that, and other key pieces of evidence, cops move in and charge Michael Johnson with murder.

From the beginning, the state's case against Michael Johnson was a roller-coaster of legal twists and turns that would lead to the state's highest court of appeals.

At the first murder trial, prosecutors introduce a surprise star witness, James McCray. McCray testifies that Michael Johnson called him to his apartment and asked him to help get rid of Phylicia's body. Then, in riveting testimony, he tells the jury that Johnson told him he strangled the 16-year-old.

"Michael Johnson confessed to him, said that the two were involved sexually, Phylicia wouldn't stop crying, and Michael Johnson put a pillow over her face and suffocated her, and he didn't know what to do with the body, and he called James for help in getting rid of the body," said Joy Lepola.

But Johnson never wavers from maintaining his innocence. His attorneys point to cracks in the prosecutor's case, including that there's no DNA evidence against him. The jury though doesn't buy Johnson's defense and finds him guilty of second-degree murder.

Finally, justice for the Barnes family, right?

"The trial was pretty much a real circus," said Russell Barnes.

Investigative reporter Joy Lepola from Fox 45 says the judge stunned the courtroom.

"We came back for sentencing and the verdict was thrown out because of James's testimony, because the state didn't turn over all the information they were supposed to to the defense, and it cost them the case," said Lepola. "It was devastating."

During Johnson's retrial, prosecutors wanted to prove there was a dangerous turning point in the relationship between the defendant and the teenager just before she died.

"The video -- I think that was a bombshell," said Lepola.

That video is described as a sex tape. On it, Michael Johnson, his brother, Phylicia and a fourth person. That person was Phylicia's older sister Deena.

"They were intoxicated. They were partially nude," said Lepola. "It's not the kind of video you would necessarily want played if she was your child."

For the first time, Phylicia Barnes's sister Deena has agreed to sit down with Crime Watch Daily to tell her side of the story. Still haunted by her sister's murder, Deena is trying to keep the focus on Phylicia.

"I feel like ultimately no matter what the public may think or what people think, we need to just keep on that path for justice for her," said Deena.

Dad Russell Barnes accuses Michael Johnson of being behind the deplorable sex video.

"I believe that video was coerced," said Russell. "The video, you know, just really showed how much this guy was really luring Phylicia, and with the girls being young and green."

Despite the charges of a heartbroken father, the jury never gets to weigh in on the video or Johnson's innocence. Due to procedural errors, the trial results in a mistrial -- but that's not the worst of it for Phylicia's family. The judge dropped all charges against Michael Johnson.

But prosecutors don't give up, eventually appealing the judge's decision to the state's highest court. And the charges of second-degree murder are reinstated against Michael Johnson. Johnson appeals that decision to the U.S. Supreme Court, and he's denied. His attorney argued double jeopardy, but it didn't stick and they lost, Joy Lepola says.

"He's going to stand trial again," said Lepola. "A third trial. But he is free now. He's out."

Michael Johnson is out, but on supervised released, and to this day maintains his innocence, denying he killed Phylicia Barnes. Crime Watch Daily tried to talk to Johnson on camera, but he couldn't be located.

"He is out right now and he is innocent until proven guilty," said Lepola. "We have to keep that in perspective, that Michael Johnson at this point is innocent."

The Barnes family knows a conviction now may be an uphill battle. But through it all, their fight for Phylicia has always been about one thing: Justice.

"I can't even describe how painful it is. But ultimately the thing that's the most painful, no matter what I've been through, is that I don't have a sister. She was my best friend, and I don't have her," said Deena Barnes.

Michael Johnson's third trial was expected to begin in March 2018. Until then he remains on supervised release.

UPDATE March 30, 2018 2:07 p.m. PT: Michael Johnson was acquitted in the murder of Phylicia Barnes.