Transcript for Darlie Routier's diary and fibers found on knife play into murder trial: Part 7
You could die for these crimes, the murder of your two sons. But the worst has already been done to me. My boys were murdered. That's the worst thing out of all this. I mean, them putting me to death is nothing compared to my boys being murdered. Every day, the crowd of spectators here gets bigger. It's really the dark side of our nature, and people are curious about those things. When I heard about Darlie's case, it didn't take me long to make up my mind that this was something that needed to be followed, and so I wanted to go see for myself. I was in the court room during the entire trial which was five weeks long. On this day, we heard the first mention of the journal routier kept. The lawyers for the state call it a suicide note, showing her depressed state of mind that led her to later kill her two sons. The defense calls that nonsense. On may 3rd, one month and three days before the murders, Darlie wrote a note in her diary to her three sons -- I hope that one day that you'll forgive me for what I'm about to do. My life has been such a hard fight for such a long time and I just cannot find the strength to keep fighting anymore. I love you three more than anything else in this world. Please do not hate me or think in any way that this is your fault. Darlie gave birth to drake eight months earlier. She was dealing with some postpartum depression at that time. That whole week, I had been having a hard time. It was like, everything. I could be watching a commercial on TV, and I would cry, it would make me cry. I started to write about the thoughts that I was having. Then I just realized, "What am I doing? This is stupid." With postpartum depression, it's pretty common for women to have suicidal thoughts. Now, that's I'm going to kill myself, not I'm going to kill someone else. And I think in her case it was self-directed. In fact, she has another entry in the journal talking about how much she loves her children and how much she wants to see them grow up and she wants them to be fine citizens contributing to society. Parents don't normally think about taking the life of their own children, and they don't. But we know also that mothers don't normally contemplate suicide, either. What is the state's theory -- that she's suicidal or homicidal? It just changes. It's, "We're gonna just portray everything bad against her we can." The crime scene itself that is the most potentially problematic for her. You've got the screen, you've got the glass. They collected, in addition to the murder weapon, a bread knife that was found in the kitchen. Charles linch, a forensic expert, looked at it microscopically and had it analyzed. He found two particles on the bread knife. One, he found fiberglass rods. Secondly, he found a rubbery compound. Why is that important? Because we go back to the screen that's been cut by this supposed intruder. There becomes a huge fight over how that screen got cut. What is the screen made of? Two things -- fiberglass rods encased in rubber indicating to us this is the knife that was actually used to cut the screen. That piece of evidence is devastating to the defense because it shows that someone inside the house cut open the screen. How in the world else could a fiber from that screen in the garage get on a bread knife in the kitchen? One notable danger with such an analysis is the fear of cross-contamination. Fibers are notoriously light. If in fact the window was dusted first, and then the knives were dusted. When they dusted the window, they could've picked up screen material, and then when they dusted the knife, could have deposited that on there. Charles linch said in an affidavit, "At the time I received this butcher block, the butcher block itself and all the knives in it had been dusted for fingerprints." So this brings up the possibility of the fingerprint person inadvertently contaminating that knife. The state's presentation of the forensic evidence at trial really didn't stand up to scrutiny. And that's why it's important to get forensics experts who are capable of doing analysis of the crime scene properly and then explaining it to the jury. Because that could make the state's whole case collapse. Now here's the problem. There's no experts from the defense to counter the prosecution experts. With every expert that you have on the state's team, you want to hear from the defense side and see if they contradict each other or say the same. I wanted to hear what they had to say. I was excited to hear what they had to say. Explain this to me. Tell me why I shouldn't believe. Give me a reason, and I simply never got one because they simply never called an expert. It was a critical mistake because the jury believed that the defense just had nothing to say. So Doug Mulder and his defense team come in, and they don't hire any other forensics experts. So when they get to trial, there's nobody there who can rebut that part of the prosecution's case. I believe there were enough flaws in the state's case that I thought we had a good chance. Especially in light that there was absolutely no reason for this woman to go haywire and kill her children. She wasn't on drugs. She wasn't on alcohol. There was no explanation for it, and there was absolutely no motive. They went out there and announced within 20 minutes there wasn't any intruder. The dye was cast, and from that point on they spent their time trying to develop a case against Darlie instead of investigate the case like they should have. Some people believe that Doug Mulder may have been a bit overly confident. I'm comfortable the evidence is coming so in so far, and I don't expect any suprises. The state's evidence against her was really quite weak and based primarily on character judgements, and he probably thought with his track record that he could just win the case on his oratory. Which might have happened if the trial had been held in Dallas, but it was held in Kerrville. I know that somebody else did this. So, if I'm put to death, I'll leave this world with a free conscience. It's not about life or death. It's about what's right and what's wrong. This was one of the biggest stories of the year, and the defense was about to reveal a bombshell about the tactics of the police with a piece of evidence that had not been revealed. My jaw almost dropped. I've never, in 20 years, ever heard a detective taking a fifth
This transcript has been automatically generated and may not be 100% accurate.