Tonight, brand-new answers in a mystery "20/20" has been following for more than a year? What really happened to the girl M in the shower? Did this mother's daughter take her own life? And convicting... See More
Tonight, brand-new answers in a mystery "20/20" has been following for more than a year? What really happened to the girl M in the shower? Did this mother's daughter take her own life? And convicting late-night surveillance. But right here tonight, on what would be her daughter's 28th birthday, investigators revealing what they think really happened. Reporter: On a Friday night, the last thing this Florida mother ever thought she'd be doing would be patrolling this small town an hour south of Tampa. But for five years now, this is exactly how Kelly Osborn has been spending every ounce of her free time. Determined to uncover what really happened at this charming beachfront hotel. Up these stairs, around the corner and behind this hotel door, where her beautiful, 22-year-old daughter Sheena was last seen alive. She was my only daughter, so she was everything to me. From the day she was born, and still to today. Reporter: And life without her? Life without her, the last five years, I am not the same person as I used to be. Look at that. Oh. Reporter: These images are from the last time Kelly would spend with her daughter, celebrating Christmas in 2008. You can see the smile on her daughter's face. You want to meet her? Reporter: Just days after this video was taken, that daughter, gone forever. How often have you gone back to look at that? I just went through it as recent as last night. It's her voice. It's the things that I don't have anymore. Reporter: Now that mother, a former real estate agent, has been thrust into a role she never imagined. A one-woman crusader investigating her own daughter's mysterious death. What was Sheena like? Sheena had a very vibrant personality. She was an amazing daughter. I could talk to her about anything. Reporter: Her love life, her hopes and dreams for the future, her babies. Her two yorkshire terriers, Chloe and encore. You know how people keep baby books? She had baby books for her dogs. Reporter: And they were not forgotten on Christmas. Oh! Reporter: Those prized dogs would be the last ones with Sheena in that hotel room. They would be left behind. And so would Sheena's friends. Do you miss Sheena? Do you still think about her? Oh, absolutely. Reporter: What do you miss? Everything. Reporter: She was a good friend? She was a very good friend, yes. Sheena owned the room the minute she walked into it. People were drawn to her. I was drawn to her. She was absolutely vivacious. Loud, obnoxious. Reporter: Vivacious and among her group of friends, Sheena was the head turner. There were always guys chasing her around, of course, which makes sense. Reporter: And the last guy to chase her would catch her. And ultimately convince her to marry him. Joe genoese, a successful contractor and builder, was trying to build a future with Sheena. They quickly fell in love. She was his everything. Sheena seemed to be happy, just kind of loving life. Reporter: But Sheena's mother was hardly sold. I was told he was 35 when I first met him. And so the minute I laid eyes on him, I said, "You are not 35." Reporter: Joe was more than twice her daughter's age. He was 42 when they met. She was just 20. And that's not all that concerned her mother. Joe already had three children and was going through a nasty divorce, not exactly this mother's idea of the perfect catch. And her daughter knew it. Sheena looked at him and said, "My mother will have a full work up on you by morning." I did some public records search. And, you know, I found some tickets. Reporter: So she wasn't kidding. No. No, she -- she really wasn't kidding. She is my only child, and it was my job to take care of her and to look out for her. Reporter: But Joe thought he was looking out for her too, paying her rent, showering her with gifts. But while there was love, their may-december romance was also stormy from the start. You know, she wasn't the type of person to back down. And he clearly wasn't the type of person to back down. So they butt heads more often than not. Reporter: And Sheena's friends tell us the relationship seemed to feed on drama. Sheena's mother was just hoping it would end on its own. You thought it would end. Yes. I was certain it was going to end. Reporter: But then, what this mother did not see coming. When did you find out that they were engaged? It was just something that just kind of showed up out there. Their engagement wasn't a huge, exciting celebration for anyone. Not her, not him. To me it seemed more like a band-aid on years of problems. Reporter: And if there were problems, that band-aid was covering them well. Because on that Christmas morning, the last time Sheena's mother would see her alive, there were smiles, laughter. And another gift to go along with that engagement ring. A diamond bracelet. That is so gorgeous. Reporter: A bracelet revealing Joe's love for Sheena, and later perhaps revealing something else. Just days later, another surprise. Joe giving Sheena a quick new year's getaway. They were headed to Bradenton beach, where they would stay in that hotel room where Sheena would last be seen alive. New years eve. 8:00 P.M. They had dinner at a restaurant across the street from the hotel. That was the restaurant where they planned on getting married. Reporter: Sheena's friends remember she was in high spirits that night, getting messages from her while she was sitting at the dinner table with Joe. I received a text message from her saying, "Happy new year." And then she had let me know that her cousin was in labor and she was beyond excited. Reporter: At the stroke of midnight, the couple back in their room. They watched fireworks off the balcony. And at midnight, of course, they had a new year's eve kiss. Reporter: But soon the sound of those fireworks, replaced by something else -- the shouting coming from their room. The couple had gotten into an argument. It got pretty heated. The people staying in the room next door ended up calling 911. Reporter: 1:47 A.M. There's two people over there just screaming and yelling, a woman and a man, at each other. Reporter: The police arrive just as Joe, the future husband, is leaving. They pass him as they head up the stairs to that room to find Sheena and her dogs. Sheena wouldn't go into too much detail about what the fight was about. Reporter: Soon the police leave. But this case was far from over. The most shocking twist was about to come next. I answer the phone, and I go "Sheena, Sheena." And there is no answer. And now I am screaming into the phone. And then all of a sudden, I hear a man's voice in the background. Reporter: What happened inside that hotel room, and the last time anyone would hear Sheena's voice again. A call to 911. He made me bleed and left
This transcript has been automatically generated and may not be 100% accurate.