Transcript for Do clues point to an intruder in Darlie Routier case? Part 5
Rowlett was known as a it's right on lake ray Hubbard where a lot of people have boats, so it was viewed I think as a kind of place to settle down. Oh, my god! We rarely if ever got a violent crime out of Rowlett. They killed my babies! Things like that don't happen in Rowlett, Texas. It's a quiet suburb. This case was going to be a challenge for the Rowlett police department. They don't have the experience that a much larger police force might have. Obviously somebody committed the crime. The question is who? There's an intriguing bloody finger print near the murders. Doesn't match Darlie, doesn't match her husband, doesn't match the kids. And so the real mystery is, whose print is that? One of the theories is that this was a botched burglary. Darin had been having trouble with his jaguar, and it had broken down that day, so the jaguar wasn't in the driveway, and Darlie's pathfinder wasn't either. If it's true this was a botched burglary, the fact that the cars weren't where they usually were could have indicated to them that the house was vacant. It turns out the day of the murder, a neighbor had reported a suspicious black car in the area. About seven or eight days before the murder, there was a black car sitting directly in front of my sidewalk. I saw this person's head turned, watching, just watching Darlie's house. I started to walk towards the car, and they pulled out very abruptly. There were these tips about a black car. In that kind of neighborhood, where everybody sort of knows everybody and at least knows each other's cars, a black car circling the streets was suspicious. I did have an occasion to see the black car again the day of the murder. The boys were murdered at 2:30 in the morning. This was probably about 12:30 or 1:00 in the afternoon. Did you go to tell the police about it? Yes, I did. I have looked at the case file in the basement of the court of criminal appeals, and I have seen nothing that made me think that the Rowlett police truly followed up on that lead. I was the one going out looking for the car with binoculars and the gun next to my seat to try to find who did this and did that for weeks because they gave up. They weren't gonna be looking. The Rowlett police department declined to comment on this case, and not just about the mystery black car, but also the other item most interesting to the defense. A sock. An adult's athletic sock was found with a small bloodstain from both boys in the alley a few doors away. The sock down the alley is probably the most important piece of evidence in this case. It was found right here by the drain. It is a good possibility the intruder thought he was throwing the sock into the drain and missed. It shows the intruder left the house and exited by the alley. The investigators were coming up with all these conclusions about what happened inside the house, and then one cop found a bloody sock. And the blood belonged to two people, Damon and Devon. How did that sock get down there? This was a big problem for the prosecution. It's always been our belief that Darlie routier placed that sock in that alley. If she had done this crime, why wouldn't she just throw it in the backyard or at the end of the driveway where it could clearly be found immediately by police? She wants to make it appear that there was an intruder who left the scene and left evidence behind. Why would Darlie dab some blood from one son and then another son on one sock, run out the back gate, run down the alley for 75 yards, wearing nothing but a t-shirt, drop the sock, run 75 yards back, a total of 150 yards. That is a long way. That's one and a half football fields of running. Why would she take a sock? Why wouldn't she have taken the butcher knife and dropped it? The notion that she could have stabbed her children, staged this crime scene, run down the alley behind the house, planted the sock, come back, slit her own throat, called 911, and the authorities showing up before the boys pass away is -- it's not credible. Once it came to light that the sock that was found down the alley was from the routier house and contained both boys' blood, what should have happened is that they should have stopped right then and said, "Okay, this may not be what we thought it was." This case is going to trial. How are you feeling today? Great, first day of the truth. We're here to win. It's hard to over estimate how intensely interested people are in this trial. The question what will the jury think?
This transcript has been automatically generated and may not be 100% accurate.