Transcript for Was Shirley Seitz Murdered?
Reporter: It's a deadly game of 20 questions. Perhaps first, does this stairway lead to accidental death or, as the family of shirley seitz believes, is it a fatal spiral to foul play? My theory is that my sister did not fall down the stairs. Reporter: Three people go to sleep in this house on a winter night in alabama. Only two wake up. The husband, dr. Mike, says maybe his wife fell down the stairs a few days earlier. But no one, not him, not her mother, who was there, saw, or for that matter, heard shirley seitz die. So what happened? No better words, slammed my sister's head against a wooden floor, or against something hard, fractured the occipital bone in the back of her head, dislocated her brain from the front of her head, and she died from the results of it. Reporter: So, is that possible? We summon two top experts. In the field of forensics, a medical examiner. In the field of falling down, a dummy. Dr. Bill manion, a private medical examiner from new jersey, has conducted thousands of autopsies. Very difficult. I mean, this is the kind of case that exasperates pathologists, because we can hear all kinds of hearsay and complaints of family members, but again, to go into court beyond a reasonable doubt this is a homicide, it would be very, very difficult. Reporter: We asked him to review shirley's case. His most important finding? Dr. Manion says, in his opinion, the autopsy shows shirley certainly did not die days after falling down the stairs, as dr. Mike suggests. She died within hours. Do you believe she would be able to carry on a conversation? Sit up -- no, I don't. Reporter: Couple days later, talk with her mother, sitting on a couch? The forensic pathologist thought this was an acute injury, and she died within an hour or two of this injury. That's my read on the autopsy. Reporter: Not days later? Not days later. That's correct. Reporter: But does that mean shirley was murdered? Not necessarily. We took dr. Manion to a house with a carpet-covered staircase, like the one where shirley lived. An important point -- shirley had only one serious injury, the blow to her head. No bruises on her back or buttocks. So, a full fall down the stairs like this is highly unlikely. It's not consistent with falling all the way down the stairs? That's correct. I would expect her to have more injuries if she fell down a flight of stairs. Reporter: In fact, dr. Manion says the only fall on the steps is that if this tripped at the bottom of the stairs. As she falls back, she strikes her head on the edge of the step. And by doing that, she sustains all the skull fractures to the back of the head. Reporter: Now, there is at least one scenario in which the autopsy and the family's theory could match. Here, in the bedroom. In a homicide scenario, you could just lift the person up and then slam them right back on the ground. Reporter: But dr. Manion has serious reservations. For one thing, shirley could have fallen accidentally and could she have within murdered with her mother right there in the house? We don't hear any boom, any commotion. The mother is there, she doesn't hear any screaming. There's no signs of a struggle. So, again, very, very difficult to say this is a homicide. Reporter: In the end, our expert, dr. Manion, says the autopsy does not have all the answers in this case. This is the kind of case where you need help from the police. Reporter: But there was an investigation by loxley police and the alabama bureau of investigation. It went on for a year and resulted in a 184 page report to the prosecutor. Although, shirley's family doesn't think much of it. I think it's a joke. I think my 10-year-old grandson could have done a better investigation than the a.B.I. Done or the loxley police department. Reporter: For one thing, the thomas family feels agents went way too easy when they interviewed dr. Mike. Do you have any suspicions as to what may have happened to her? The only thing I can rationalize, based on the way she was feeling, and what the autopsy report said, was she had to have slipped down the steps and hit her head. Reporter: The police do ask mike whether he might have accidentally done something to shirley. And you're a chiropractor, right? Yes. You said you worked on your wife? Yes. Do you think something might have happened when you were adjusting her or something? Or -- I've adjusted thousands upon thousands of people, and I've never had -- I mean, my entire career I've never had any malpractice suits, never had any injuries. Other than a couple of broken ribs on some old ladies. Reporter: When the a.B.I. Ended its investigation, the county prosecutor took no action. The thomas family is furious. Why has nothing happened? It's because someone does not want to do their job. They have proven to me -- I'm talking about the baldwin county district attorney's office. That they do not want to work hard on this case. Reporter: They hire birmingham attorney henry frohsin, and nearly two years after shirley's death, they sued dr. Mike. The lawsuit alleges the deceased was killed through the actions -- by the actions, of michael wohlschlaeger. Reporter: Answering the suit, dr. Mike flatly denies having a hand in shirley's death. But he didn't even need to bother. That's because the lawsuit was dead on arrival. There was one factor that we did not know and didn't appreciate. Reporter: It turns out shirley's daughter had agreed not to sue mike, and sure enough, the judge threw out the lawsuit, leaving any legal action in the hands of the criminal justice system.
This transcript has been automatically generated and may not be 100% accurate.