We're going to turn next to another baffling mystery. He thought his crime would never catch up to him. What happens next is nearly impossible to believe. See if you can solve the case as fast as this... See More
We're going to turn next to another baffling mystery. He thought his crime would never catch up to him. What happens next is nearly impossible to believe. See if you can solve the case as fast as this private investigator, right in front of our "20/20" cameras. Reporter: Drive along interstate-10 long enough and eventually you'll run into Beaumont, Texas, home to big steer, big oil, and one big mystery. It happened here at the elegant hotel, not exactly the four seasons, although Greg Fleniken didn't mind it. He liked it here. But on September 15th, 2010, the oilman and devoted husband, seen here on hotel security footage, walked in and headed to his room. Hours later, he would check out in a body bag. He'll be missed and, by us that loved him. And such a good guy. Reporter: When the sun rises that morning in Texas, Greg Fleniken does not. Detective Scott apple is sent in. Reporter: So you remember getting that call that Thursday morning? Yes, sir. And it just came in as a, a deceased person at one of our local hotels. Reporter: With no clear signs of foul play, at first, the veteran detective thought this was going to be routine. You'd heard heavy smoker. Mm-hmm. Reporter: Had been a drinker in the past. Yes. Reporter: A lot at stake. Yes. Reporter: So it wasn't. And never saw a doctor. Reporter: So you thought this guy might have had a heart attack? That's what we had just figured it must be at the scene. Reporter: Detective apple heads to room 348. And inside that door, a 55-year-old man face down on the carpet. You open the door -- Right. Reporter: And you see -- Greg Fleniken on the floor. Reporter: Right there. Yes, sir. Reporter: In fact, there's nothing to suggest this is a crime scene at all. Fleniken's cash is still in the room, candy and cigarettes on the bed, a soda on the table. At first glance, it looks like this must have been death from natural causes. Reporter: This room was his home away from home. Right. Reporter: Greg Fleniken had lived a good life. He had a great wife, success in the oil business, many friends and few, if any, enemies. He'd give you the, not only the shirt off his back but the shoes off his feet. Reporter: Friend miles martin met Greg in the fourth grade in Louisiana, and those boys from the bayou maintained a friendship ever since. I thought, well, those damn cigarettes. They finally, you know, they snuck up on him. Reporter: Inside room 348, detective apple says his initial investigation indicated that the victim had a regular, rather humdrum routine. Fleniken, at home with his wife in Louisiana on the weekends, but during the week, he would live out of a small suitcase here at the elegant doing pretty much the same thing every night. He'd set up, and that was his routine for the evenings. He had his phone to communicate with his wife or business. Reporter: It seemed the travelmang oil had settled in R the evening. He just ordered "Iron man 2" on pay-per-view. He, he was a creature of habit, and that was his routine. Reporter: But this cajun cowboy had taken his last ride to Texas. All that was needed now was the medical examiner to list a cause of death. Everyone thought the case would be closed. Reporter: So his body is taken away to the medical examiner? Right. And pending if he can just essentially confirm that it was a heart attack. Reporter: Dr. Tommy brown, a fast-talking texan, was the medical examiner here. He was about to add a shocking twist to the case. Brown would find that Fleniken's body suffered severe internal damage, broken ribs, lacerations to the liver and scrotum, and a hole in his heart, all caused by blunt force trauma. But from what, and from whom? Reporter: Nothing had happened to him at work? Nothing had fallen over on top of him in that room? There was no accident or anything happened to him on the way to the hotel, rather. Reporter: No accident. In fact, the medical examiner was now ruling this death a homicide, convinced that someone had killed him. Reporter: Did you ever think the ruling would be homicide? No. Reporter: Detective apple is a trained sniper, and he was now taking aim at multiple theories. Could he have been seeing another woman or maybe there's a jealous husband? You know, but that's not Greg. Reporter: Detective apple wants to know if anything odd happened at the elegant hotel that night. But aside from a circuit breaker needing repair in Fleniken's room at 7:30 P.M., it was business as usual. Apple questions the hotel guests in nearby rooms, but their answers offer little help. And who were they? They were some electricians out of Wisconsin down here working at one of our refineries. Reporter: That's electrician Tim Steinmetz and his colleague lance Mueller entering the hotel with beer. They're later joined by fellow electrician Trent pasano. You asked them had they heard anything that night? They said, "Oh, we hardly ever saw him. We don't know." Reporter: Fleniken's cell phone records show nothing but a dutiful husband. It was pristine. It was his business stuff, his stuff with his wife, family. That was it. Reporter: The detective works tirelessly, but all he has is a dead guy, a bunch of dead ends, and as the days drag into weeks, a very angry widow. She would vent and yell at me. I can take that. I understand that. Reporter: With the investigation seemingly losing steam in Texas, a grieving and frustrated Susie Fleniken is desperate for answers. We need to know if anybody heard anything, saw anything, knows anything. Things were basically stalled. And until she, you know, she took control over the situation at that point, and she hired her own investigator. Reporter: And not just any investigator, but a former New York cop and federal agent now living in Florida. His name is ken Brennan, a man with a passion for crime solving, motorcycles, fine cigars and, yes, golf. Reporter: So your phone rings. So my phone rings. Reporter: Do you remember where you were? Yeah, I was on a golf course. Hello? Reporter: And what does she say? That it's kind of getting to be a cold case going on the back burner. And she wanted to see if I could find out who killed her husband. The private eye is off to Beaumont, Texas, to get a better feel for the case. Reporter: First thing you want to do is meet with the lead detective on the case. Scott's one of those kind of guys. He's just a real cop's cop. And we hit it off really well the first night I met him. I'll start on the carpet. You can start in the back. Reporter: This dynamic duo starting from scratch reviewing the case file, going through the hotel room like housekeeping, even taking a closer look at the victim. Reporter: Alcohol? No. Reporter: Drugs. No. Reporter: Pills? No. Reporter: Nothing. Nothing. The only thing the guy did was smoke and eat candy bars. That was it. Reporter: With more than $1,000 in cash still left in that hotel room, they rule out robbery. So instead, the investigators are now examining the security camera video, the broken circuit, some photographs. But none of them really feel like clues. There's simply no compelling theory, that is until ken Brennan has a classic a-ha moment and then shared it with his partner, that detective stumped. I said, "I think I know how this guy died." I said, "I think I know when it happened." I said, "I think I know who killed him, and I think I know how we're gonna catch him."
This transcript has been automatically generated and may not be 100% accurate.