Transcript for 'Making a Murderer' Prosecutor, Filmmakers on Evidence Left Out: Part 1

Perhaps not since the O.J. Trial has America been so fixated on a criminal case. The new netflix documentary series "Making a murderer" has millions of us dissecting and debating the bizarre story of Steven Avery. Tonight we're going to take you to the place where it all unfolded to meet the central players and examine the evidence the documentary left out. Here in this tiny frozen over speck on the map the scene of a crime that has become a national obsession. Now to a hot trend ticking over TV -- A netflix do you want tear series captivated the nation -- Reporter: "Making the murderer" explores the strange odyssey of Steven Avery, 2007 sentenced to life in prison without parole for the murder of a 25-year-old woman. In the four weeks since the series was released a frenzy of binge watching fueling conspiracy theories and outrage. Both among those who think Avery was wrongly convicted, framed even, and those who think the documentary is at best misleading. Wake up, netflix! Reporter: I've come to machlt anitua county, Wisconsin. After having absorbed ten hours of this story, to be here is strange. Reporter: To see if I can wrap my head around the mystery at the heart of this saga, is Steven Avery a killer? The documentary begins in 2003. Steven Avery had just been released from prison after serving 18 years for a rape he didn't commit. I'm just glad you're home, honey. Reporter: Exonerated by DNA evidence. Then two years later, just as Avery is in the middle of a multi-million dollar lawsuit against the authorities who put him away, those very same authorities charge him with murder. I didn't do it, I'm innocent. Reporter: The once-celebrated face of wrongful convictions now accused of killing and dismembering Teresa Hallback, a young photographer who had come to the Avery family scrap yard to take pictures of a vehicle he was selling. Her charred remains found near Avery's trailer. It's hard enough going to prison for something you didn't do, then you got to do it all over again? Reporter: The story caught the attention of two aspiring filmmakers, Moira demos and laura richard, who packed and up moved to Wisconsin. We were there because we wanted to ask bigger questions about the system. Is it guilty beyond a reasonable doubt, is the process fair, can we trust the verdict? Reporter: They devoted a decade to the project, gaining extraordinary access to the main players, from Avery who spoke to them by phone from jail -- They wouldn't look at nobody else. They're paying all their attention to me. Reporter: To his family waiting on the outside. They don't care. They'll take an innocent man and make him guilty and that's what they're doing right now. Reporter: To Avery's defense team who argued that their client was being railroaded, alleging that the local sheriffs, furious about Avery's lawsuit, framed him. How many times will Steven Avery be charged with rapes he didn't commit? Reporter: The filmmakers capture key moments. This is a red letter day for the defense. Reporter: When Avery's attorneys discover a vial of their client's blood, evidence from his first wrongful conviction, appears to have been tampered with. Some officer went into that file, took a sample of Steven of Ary's blood, and planted it. Reporter: Also in the film we see that the lawyers believe the police planted something else. The key to Teresa Hallback's Toyota. When you came into that bedroom the first time, there was no key on the floor, was there? That's correct. Reporter: It wasn't discovered until the seventh search of Avery's bedroom. And it just so happened to have been found by two members of the local sheriff's department who had recently been deposed in Avery's civil lawsuit. As you sit here today, all these years later, do you still believe the department or members of department framed Steven Avery? I'm still left with real reason to suspect that. Reporter: "Making a murderer" transformed Avery's former defense attorney, the eloquent and emotional dean strang, into a digital-age folk hero. Redemption will have to wait as it often does in human affairs. Reporter: We met him in his office where he said the case still troubles him nine years later. I want him out of prison. I really am haunted by the concern that he's sitting there innocent. Reporter: In stark opposition to strang, some on the internet have identified a villain, the man who put Avery away. Prosecutor ken Kratz. When we arrived he showed us nasty e-mails -- I hope your daughter gets raped and murdered. Reporter: And threatening voice mails. I'm going to do everything in my power to free Steve Avery, then I'm coming after you. Reporter: Kratz blames the filmmakers. This is not a documentary at all. It's still a defense advocacy piece of what they pick and choose, causes only one reaction and only one conclusion, that Mr. Avery was innocent. Reporter: Kratz points out there was a mountain of evidence against Avery. Hallback's remains and her car both found on his property. And Avery's DNA found on that key which Kratz says was only discovered on the seventh search because that's when it fell out of the back of this bookshelf. Kratz also points out the jury rejected the framing defense. We the jury find the defendant, Stephen a. Avery, guilty of first degree intentional homicide. Reporter: In part because they saw vital pieces of evidence the documentarians excluded, including that Steven Avery's DNA from sweat was also found on the hood latch of Teresa Hallback's Rav 4. Why is that important? Because you can't plant, first of all, sweat. How do you leave that out of the documentary? If his hand was there, why didn't you find fingerprints? They're looking for DNA. That's what was looked for and that's what they found. They didn't look for fingerprints? I don't know. I would imagine it would be an important thing to know. You're alleging that he drove this vehicle and hid it. I would imagine his fingerprints would be all over the thing. They might have. I don't have the case file. Reporter: According to krat Kratz, perhaps the most damning omission in the series, Avery made three calls to Hallback the day she went missing and also requested her by name to come photograph the van he was selling. Steven ofavery did not just come upon Teresa Hallback by accident. He targeted her. If as you say Avery was targeting Teresa Hallback, why would he call and specifically request her? Wouldn't that just be a trail right back to him? He believed, at least my theory is, that using a different name and a different phone number was good enough. That would require him to be pretty stupid. Okay. Reporter: We ran Kratz's complaints by the filmmaker. His argument that is you left out some key things. And that in the end, it was pro-steven Avery misrepresentation. I disagree. It would be impossible for us to include all of the evidence that was presented at trial. That's called a trial. What we made was a documentary. Kratz himself later said that he presented a circumstantial forensic science case and that's what we tried to show in the documentary. He did not have direct evidence of Steven Avery's guilt. I'm sure if he did he would have used it. Reporter: Sometimes lost in the uproar over the Avery case, that he was not the only one convicted for the murder of Teresa Hallback. His 16-year-old nephew was also sent to prison, in part because of what he told police. Who shot her in the head? He did. Why didn't you tell us that? I couldn't think of it. Reporter: Was this admission coerced? The controversial confession up next. And the surprising twist that landed the winning prosecutor back in the headlines. Plus our investigation leads us down that dead-end road. That's an Avery -- Reporter: When "Nightline" returns.

This transcript has been automatically generated and may not be 100% accurate.