Reporter: If you've ever met private investigator ken Brennan, no doubt you remember him. I like to say ken Brennan's just an honest, straightforward guy. Kind of easy to read. What you see is what... See More
Reporter: If you've ever met private investigator ken Brennan, no doubt you remember him. I like to say ken Brennan's just an honest, straightforward guy. Kind of easy to read. What you see is what you get. Reporter: When he's not riding motorcycles or pumping iron, he spends his time doing what he really enjoys -- solving crimes. What is it about this kind of work that attracts you? Who doesn't like catching bad guys? You know, it's -- rewarding in itself and something I've always, you know, enjoyed doing, even when I was a youngster. Reporter: After decades in law enforcement, first, as a police officer on Long Island, New York, then, as a Dea agent in Florida -- I've been working in law enforcement since 1975. Reporter: Brennan now works as a private investigator. It is true that you've never taken a sick day? No, I really haven't. When you enjoy what you're doing, it never seems like a job then. Reporter: It seems that Brennan has a particular devotion to solving sex crimes. To this day, he still carries around the arrest file for the first sexual predator he ever interrogated. They're the worst crimes, I feel, of all of them. And there's too many of them out there that aren't solved. Reporter: By November 2005, nearly nine months after inn budnytska had been attacked, the case of the airport regency rapist was on a slow boat to the cold case file. Were you thinking at any point that this case is just unsolvable? I don't like to think that and I really didn't think that. I always kept hope that something would eventually solve the case. I don't like to give up. Reporter: In the aftermath of her attack, inn has filed a multimillion dollar lawsuit against the hotel. We felt they could have had better security. They weren't monitoring the security cameras correctly. They let an assailant onto the grounds of the hotel. Where were the security. Why and how somebody would get into your room without the key. How? Reporter: The hotels denies any wrongdoing and needs an investigator to disprove the woman's claims. That's where ken Brennan comes in. We have to clear the responsibility of the hotel. What happened in our hotel, it can happen in the best hotel in the world. The biggest clearing thing was that nobody really knew how it occurred. Everybody loves a good mystery, you know? Everybody wants to be the -- the ability to solve something that somebody wasn't able to do before. Of course that would be rewarding. Reporter: Like many city detectives, Alan Foote isn't wild about private eyes. What did you think of ken Brennan when you first laid eyes on him? This guy is going to drive me nuts. Reporter: But Brennan isn't your typical hired gun and Foote has a hunch, he's somehow different. Maybe it's his experience. Maybe his straight-talk. Most police officers don't like P.I.S. Reporter: How did you get him to accept and to trust you? I said, "Hey, listen, Allen. I'm not going to mess up your case, you know. I'm not going to screw it up. If we're going to solve this thing, we're going to be a team. We're going to work it together." Reporter: You don't like P.I.S, but yet, you trusted him. Why? There was just something about him that was honest. I really felt that he was going to do what he said. Reporter: One of the first things Foote shares is the DNA results that have come back on his first two suspects. Inna's friend, peter dimloueas, and the night manager, George Perez. Both negative. I was cleared of any charges, because, apparently, I had nothing to do with it. Reporter: So, those two are eliminated. But parent unanimo but Brennan shares the detective's suspicions about inn herself. So he goes back to the cul-de-sac and interviews the neighbors. Where exactly were they across the street? Reporter: He combs through inna's background. Even puts her under surveillance. There was nothing indicative of her being a prostitute. Reporter: So, you ruled that out? I ruled that out pretty readily, in the very beginning of the investigation. I knew who I was. I know who I am. Reporter: Still, there's that strange 17-minute gap between the security camera at the elevator and the key card swipe at inna's door. Where had she been? I said, well, listen, did anybody take the time to find out if the timestamp on the security cameras, if that matched the timestamp on the card access system? Which nobody ever did. And it turns out, when we did do that, there was a 17-minute lapse. Reporter: So, that mystery is solved? So that mystery was solved. Reporter: Now, Brennan has another hunch. That the entire incident could have been an elaborate scheme to shake down the hotel. I thought that in this particular case, that it might have been a possibility that she could have been involved in some kind of a scam. That the whole purpose was to try to obtain money from either the insurance company or from the hotel itself. It happens all over the world. Reporter: Because she was filing a lawsuit against the hotel, a big lawsuit. Exactly, and it -- she didn't make a very good witness. She flip-flopped on a lot of statements that she'd given to police. It just make me to laugh. I mean, I don't know from where did they take this and who came up with this idea? Reporter: The only thing Brennan has resembling a witness statement is inna's muddled memory about being attacked in her room by a number of men. So, he takes a novel approach to that evidence -- he throws it out. The last thing that I put faith in is a victim's statement. Because when something traumatic happens to them, their memory becomes distorted. The thing you have to base your decisions on and where you're headed in an investigation is based on the evidence. What -- what evidence do you have at that particular time? Reporter: That means one thing -- I believe that no matter what happened, it was going to be in video tape. Reporter: How much footage were you going to have to comb through? Because there were a lot of cameras in the hotel, right? Hours upon hours upon hours upon hours. It was extensive. That's something that you have to go through almost frame by frame. Reporter: Frame by frame? It's a countless amount of hours. Reporter: Now, every guest is a potential suspect. Hundreds of them. As well as every desk assistant, bellman and janitor. You have to watch each and every frame on every video because we originally were look ING for possibly an employee that might've moved one of the cameras out, out of the location it was originally at, and maybe, possibly, snuck the victim past that way. Reporter: Slowly, but surely, Brennan eliminates every suspect. Everybody but one. She goes out. On the video, she goes out of the hotel early in the morning. When she returns, there's a big, large, black man standing with her and she just has a quick conversation with him, they get onto the elevator together. Trying to look to see, do they look like they know each other? Because, again, I'm thinking this could still possibly be a scam or something. And after you keep reviewing it, it doesn't look like they have any kind of familiarity. He's just offering her to walk in front. She's not acknowledging him like she knows him. Reporter: Brennan has now rejected his own conspiracy theory. It didn't appear to be staged at all. Reporter: But his focus on this man seems like just as much of a stretch. Later on during the video, he's seen exiting the elevator, so, we follow him to another camera and he goes off the property. While I'm watching the video of this person, this large person, black person coming off the elevator, I went through this frame at a time. Reporter: And now, in extreme slowotion, ken Brennan is about to crack the case. This was the defining moment in the whole case. Right here. Reporter: Don't go anywhere. So what do you think ken Brennan has spotted on that surveillance footage? Any guesses? We're live tweeting tonight, so, let us know, using #abc2020. We'll be right back.
This transcript has been automatically generated and may not be 100% accurate.