Transcript for Retired NYPD detective examines woman's death, says case worth another look: Part 4
Reporter: Seven years after the death of Rebecca zahau, the mystery lingers. How could this beautiful young woman have committed suicide fully naked, with her feet bound and her hands tied behind her back? I think the interest in this case remains years and years and years later. I hope somebody looks at this case again. Reporter: It turns out someone is re-examining the case, 3,000 miles away from the sunny beaches in Coronado, here in the city that never sleeps -- New York. His name is Herman Weisberg, and he's a seasoned private investigator in New York City with lots of high profile cases under his belt. Weisberg is also a retired veteran NYPD detective. "20/20" retained Weisberg to do a deep dive into the Rebecca zahau case. He examined the case file of the San Diego sheriff's department, poring over the detailed investigation. It's a mystery. And I don't like mysteries. And this one begs for a bunch of questions to be answered. Reporter: Have you seen anything as perplexing as this one? Perplexing? No. Reporter: Weisberg says one of his biggest concerns about the San Diego investigation was over the forensic analysis. Remember, one reason investigators concluded that Rebecca wasn't murdered was because only her fingerprints and DNA were found at the scene. They say science doesn't lie, that's why I'm so confident here. There's a lot of questions about what should have been found and what wasn't found there. Reporter: For instance, Weisberg wondering how it could be possible that there was no trace of Adam shacknai's DNA and fingerprints, considering that he says he cut down the rope Rebecca was hanging from and administered cpr. I just put mouth over hers, I said, "-- it, it's my brother's girlfriend, I'll do it," and I did it." Reporter: Is it odd to you that his DNA was not found on the rope? Was not found on Rebecca's body? The fact that it's not there means it wasn't collected properly or it wasn't analyzed properly or both. Reporter: Weisberg is also troubled by the investigators' conclusion that Rebecca was able to hang herself with her hands tied behind her back. When you look at the way her hands and her feet were bound, what are your observations? This is a very intricate way to bind anyone. Looks to me like it takes some knowledge. Reporter: But remember, the San Diego sheriff showed how a woman was able to bind her hands behind her back in way they said was similar to Rebecca. Best we can tell, there was nothing, no particular expertise required. Reporter: No particular expertise? To find out, Weisberg met up with a rope tying expert -- Nice to meet you. Pleasure. Reporter: Greg hanchrow, a New York harbor port captain and a former tugboat captain. Weisberg showed him the bindings on Rebecca's wis. Could these be done by somebody like me who's never really had an experience with tying knots for these purposes? I don't think a layman that has never secured stuff before would come up with that kind of lashing and knot to tie it. But it doesn't have to be exclusively from a maritime background. Let's just take this, and show me how you would do it. Reporter: Just like the woman in the sheriff's video, hanchrow was able to tie his hands behind his back using the same type of rope on Rebecca's wrists -- but even he found the process to be awkward. So now I'm gonna try to tighten the knot. I can't imagine this happening without a lot of practice. This is a very elaborate situation. It's pretty elaborate. I would say to a layperson, that's pretty out of the ordinary. When you lay a lashing like that, there's experience behind that. Reporter: But could Rebecca have had that kind of rope tying experience? After all, Jonah shacknai says she often went out with him on his boat in Coronado. Rebecca, who was a really good athlete, would sort of jump off the boat and tie it down. Certainly she tied down our boat on dozens of occasions. Reporter: But our rope-tying expert told us that the type of bindings found on Rebecca wouldn't be used to tie down a boat. Weisberg also has questions about what the San Diego investigators say happened on Rebecca's balcony. Remember -- they concluded that because only Rebecca's footprints were found on the balcony, she must have thrown herself off. To see just how likely that scenario could be, Weisberg brought me to this balcony, roughly the same dimensions as the one at spreckels mansion. So from this point forward, it's roughly 24 inches or two feet. The rail from balcony, it's right around the same, within an inch. Reporter: She would have hopped. Hopped. It's extremely difficult to get yourself in that position while you're tied up, while you're wearing a gag. Reporter: Naked. Naked. And your ankles are bound. Reporter: And pretty gruesome, too. You would have to throw yourself over headfirst, bound and gagged. But could an attacker have thrown Rebecca off the balcony without actually stepping on it and leaving incriminating footprints? To find out if that was possible, we brought in this dummy. Is this dummy similar dimension? Similar. A little bit heavier. We erred on the side of heavier rather than lighter. It's a really good way of determining if this is possible for me to do this. Reporter: All right, let's see if you can do it. So it is possible. It's not easy. Reporter: It's not easy, but it's possible for you to have not stepped out there and positioned the body and thrown the body over. Sure. It shows that someone could have taken Rebecca's body, put it down, leaned it against that rail and shoved it off. Reporter: In the end, Weisberg says his evaluation raises serious questions about the San Diego sheriff's department's findings in the Rebecca zahau case. Just looking at it with a fresh pair of eyes, does the evidence point to a suicide in your view? I'm not completely satisfied that this was a suicide. But there's also not enough evidence at this point to show that this was a homicide. I think there's a lot of evidence that has to be re-examined. I think that we really need a fresh look at what wasn't done here. Reporter: The San Diego sheriff's department declined our request for an interview, but told us they are always willing to look at any new evidence or information that comes to light.
This transcript has been automatically generated and may not be 100% accurate.