Reporter: On a clear September day in 2008 here in Albuquerque, New Mexico, 17-year-old Brittani Marcell was walking in her door when she was brutally beaten on the head by a man with a shovel. As detectives at the scene are piecing together the evidence, doctors are trying desperately to save Brittani's life. They place her in a medically-induced coma for two weeks. But she does wake up. This is Brittani Marcell today. She's now 27 years old and says when she came to at the hospital, she had no memory of that terrifying day. Was it scary not being able to remember anything? It was scary after they told me what had happened. Reporter: So you at first didn't know why you were in the hospital? Right. I thought I was in a bad car accident. They're like, "That's far from what happened to you. You were beaten traumatically, with a shovel, in your house." It was hard to take in. Very. They had to go in and remove part of the lower left temporal lobe of her brain. It's about a nickel size. They removed probably three by four, four by three part of her skull. Reporter: Wow. Implanted that in her abdomen. Reporter: That's a pretty big size. Right. Reporter: That's a big chunk of skull. They cut her abdomen and put it inside her abdomen. It healed inside of her abdomen. So she had to wear this helmet. Reporter: Eventually doctors would put the piece of healed skull back in Brittani's head. They cover the mirrors in her hospital room, afraid of how she'll react to her own reflection. Her close-knit family spends every waking moment at her bedside. We tried to get her out of bed and seeing her in the wheelchair and she was drooling all over herself, I was like, "What happened to my sister? I'm pushing her in a wheelchair and she physically cannot keep her head up. How could somebody do this to my sister?" Reporter: What was it like to see your daughter have to learn the things that you've already -- you'd already taught her when she was a toddler? It was very difficult. Reporter: Eating, walking. You'd cheer her on, you know, "You can do it. Reporter: What were the doctors telling you? Hat she was going to have memory loss and it could be long term, it could be short term. Just don't get your hopes up too high. Reporter: But hope is all the family has to hold on to, that Brittani can recover physically and maybe recover memories that could lead to a suspect. Meanwhile, detectives still have that single drop of blood. They run it through the national DNA database hoping for a match, but no luck. As the months pass, friends and family put up billboards and turn to the media for help. The person that did this to Brittani still hasn't been caught. We wanted to let people know that he's still out there. The tips kept coming in. I'd call the detective probably every day, and the reward fund was going up and nobody. Reporter: The Marcell home, once filled with so many happy memories, now too painful for Diane to go back to. You never lived in that house again? I never lived there again. I couldn't do it. There was no question. Reporter: Instead she splits her time between nursing her badly injured daughter back to life and being her own investigator on the case. Reporter: You were suspicious of basically everybody. I was. Everybody was on my radar, I don't care if it was a friend, a neighbor, a co-worker, I want to know who did it and I wasn't gonna trust you. We all thought it was someone that she knew, just based on how passionate of a crime it was. Reporter: But figuring out who would want to hurt the high school senior with the beautiful smile wasn't easy. She had no enemies. At points, I thought it was mistaken identity. Reporter: Diane begins to think perhaps she was actually the intended target. After all, she did go through a contentious divorce just six years earlier. My ex-husband, he was a bitter person after the divorce and one of my daughters said, "Do you think dad could have done this?" Reporter: He wasn't just bitter, he was violent during your marriage. I got out of a marriage because of domestic violence. Reporter: Did you know that he had been violent towards her? I did. As a kid I -- I distinctly remember that. And it was something you shouldn't remember. Maybe he, you know, had hired somebody at one point because he didn't like me anymore and they got the wrong person. Reporter: But her suspicion would soon turn into more disappointment. We were able to speak to him and kind of figure out where he was, anybody that he might have been dealing with, and that information never really panned out. Reporter: Brittani's dad is officially cleared. And for ten months, Brittani continues to improve, relearning to walk, talk and swallow on her own, eventually even earning her high school diploma from a rehab facility where she lives after leaving the hospital. As you're seeing this amazing progress with your daughter in her rehabilitation, you're not seeing progress in the investigation. Right. I just thought, "Well, they're not going to find him." You did start to worry that the case may not ever be solved. Reporter: David Waymire is a deputy district attorney in Albuquerque. Unless Brittani Marcell recovered memories of the attacker, that the one and only piece of evidence was a single blood drop. Reporter: That's right. The whole case hinges one drop of blood. So two years after the attack, Waymire makes a bold move. To get around the statute of limitations, he indicts the DNA itself from that drop of blood, referring to it as John doe. He said, "Whenever this person's arrested, his name will attach to it. Right now, is going to be a John doe, because, you know, we know it's a man." Reporter: But a name is what investigators needed to close this case, and Diane Marcell thinks a new detective could breathe new life into this case. Talked to the sergeant, I said, "When can we get a new set of eyes on it?" She said, "I got somebody for you." She told me, "I'm going to find the person who did this." Reporter: And her out of the box approach is about to turn this cold case red hot. She said, "Well, let's just try something new."