The mysterious Rebecca Zahau case as wrongful death suit heads to civil court

Zahau was found naked, bound and hanging at a historic California mansion in 2011 and authorities ruled her death as suicide.
4:36 | 03/02/18

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Transcript for The mysterious Rebecca Zahau case as wrongful death suit heads to civil court
new developments in that mysterious death, a woman's body found at her boyfriend's San Diego mansion. His brother is the focus of a wrongful death civil suit now under way and Amy has that story. It is bizarre. The death was originally ruled as a suicide. Officially ruled as a suicide but her family doesn't believe it and taking her former boyfriend's brother to court looking for some measure of justice. The body of a woman was found hanging. A mysterious death at this million dollar mansion left an exclusive southern California beach community reeling. 32-year-old Rebecca zahau was found naked and hanging from the second story balcony. Reporter: Her body discovered hanging from a balcony. Reporter: 32-year-old Rebecca zahau found dead happening in the sprawling 27-room home of her 54-year-old boyfriend pharmaceutical tycoon Jonas shacknai coming on the death of another terrible tragedy. She was the only adult home when her boyfriend's 6-year-old son max plunged over a staircase. He laid dying in the hospital when authorities found her body. The sheriff's office struggled to determine whether zahau committed suicide over max's dent or if she was murdered as possible retribution for the boy's demise. The scene was pretty suspicious. We are not ready to determine whether it was a suicide or a criminal act. Reporter: Her boyfriend's brother Adam shacknai a tugboat captain from Tennessee was visiting the family. He found Rebecca's life also body that day bound and gagged, cut her down and called 911. 911 imagine. A girl hung herself. Is she still alive? I don't think so. Reporter: After a seven-week investigation they ruled her death a suicide. But her family never buying it. This morning Adam shacknai faces a civil jury over her death. My sister was murdered. It is pretty obvious she was murdered. Reporter: But Adam shacknai's attorney says his attorney is not guilty and there's no evidence connecting him to zahau's death. Her DNA the only one police found at the scene. They concluded she committed suicide. Reporter: For the first time her boyfriend and brother speaking out. She was an enthusiastic person. Defending his brother. He wakes up and his life is changed forever not because of anything he did but because where he was, the wrong place at the wrong time. Reporter: Authorities had cleared Adam shacknai of any involvement in Rebecca's death years ago but the zahau family is asking for an unspecified amount of money in the lawsuit. It is a civil case, of course, and Adam shacknai faces no criminal charges and cannot be sentenced to prison in this case. Robin. All right. Amy, thank you. Our senior legal analyst sunny Hostin is here investigating the story for "20/20" on tonight. Oh, my goodness. When you look at this and there are many reasons why the family rejects the idea of suicide. Yes, and I met with Rebecca's sister. She said her sister was full of life. She was devoutly religious and she would have never committed suicide, especially in this fashion. I investigated it and spoke to a lot of people, robin. They said they've never a woman commit suicide in this fashion which would be getting completely naked, gagging herself, binding herself with nautical knots and throwing herself over a balcony. It's the most odd way to -- that one would commit suicide. You have spent a lot of time and talked to a lot of people and hired an outside consultant in we did. We hired an outside investigator. Former NYPD detective. He H the same questions. Why would a woman commit suicide this way? Is it possible for her to commit -- would it have been possible to commit suicide this way and you'll see it's very unusual and he actually -- we sort of reenacted the alleged suicide and he has come to the conclusion it's quite possible that someone threw her over the balcony. Yeah. You think there is a possibility that this may turn into a criminal investigation. You know, the sheriff's department in Coronado said they are willing to re-open an investigation if they get new evidence and so perhaps they'll be watching tonight. Oh, my gosh. It just is an eye opener and your heart goes out to her family. Just an incredible, incredible case. We will be watching tonight. You can see more of the case tonight at "20/20" at 10:00 P.M. Eastern and see this woman

This transcript has been automatically generated and may not be 100% accurate.

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