Kathleen Zellner did not wait to be asked. “The blood spatter was really suspicious,” volunteered the Chicago lawyer who specialises in getting the innocent out of prison – predicting a question about when, as she watched the Netflix global phenomenon Making a Murderer, she decided to take up the case of Steven Avery.

“Watching his facial expressions during the trial, I thought there was a strong possibility he was innocent. I was once appointed on a serial killer’s case. I spent hundreds of hours with somebody who really was a killer and a sociopath and now I’ve spent thousands of hours with people who are innocent. I know the difference.”

Avery is serving life without parole for the 2005 murder of a young photographer, Teresa Halbach, at his sprawling Wisconsin auto-salvage yard. At the time of the killing, he was only a couple of years out of prison and expecting a large compensation payout after DNA exonerated him of a rape for which he wrongly served nearly two decades.

Mug’s game: Steven Avery in custody in July 1985. Photograph: Netflix

The murder trial attracted a bit of attention because of the strange twist of Avery being cleared of one serious crime after so much time only to be quickly convicted of another. But the prosecutor, Ken Kratz, didn’t take much note of two filmmakers, Laura Ricciardi and Moira Demos, following the case.

Avery and his 16-year-old nephew, Brendan Dassey, were found guilty of murdering Halbach and burning her body, and were locked away in 2007. No more than another tragedy, it made a faint blip on the national consciousness before fading into the past, along with the thousands of killings the American courts cycle through every year.

‘The judicial system is rigged against undoing convictions’

But then Making a Murderer, Ricciardi and Demos’s 10-part documentary series telling the story of Avery’s wrongful arrest for rape and trial for Halbach’s murder, burst on to the scene in 2015, a decade after the killing, and quickly became a sensation. Its Zola-esque twists and turns seemed to put everyone on trial. The Manitowoc County Sheriff officers, suspected of framing Avery because he was suing the police department and county for millions over his wrongful rape conviction. Kratz, who appeared more intent on a guilty verdict than justice as he offered lurid theories about the murder that got a lot of public attention, including from potential jurors, but were never aired at the trial. And the courts, for arcane ways and narrow thinking that seemed more about protecting the system than finding the truth.

As the doubts and alleged conspiracies piled up, perhaps the most shocking revelation was of detectives coercing a confession from Dassey. Police video of the interrogation shows that after hours of questioning without his mother or a lawyer present, the teenager, who is routinely described as “slow”, admits to joining Avery in the gruesome rape and murder of Halbach. But the practice of detectives telling Dassey key information, instead of him telling them, and repeatedly suggesting that if he just said what they wanted to hear, he would go home, badly tainted the whole process. As soon as the detectives leave, and Dassey is able to speak to his mother alone, he promptly recants, telling her he only confessed because the officers got inside his head. Kratz told the jury at Dassey’s trial that an innocent person does not confess to a crime he has not committed, even though there is a long history of people doing exactly that.

Zellner watched all this unfold at the same time as the rest of the world but, unlike the furious debates that swiftly washed around the internet as theories were aired and shot down, she came at it with the eye of a lawyer with a long record of challenging supposedly solid convictions. The state had stopped paying for Avery to have an attorney years earlier and he was attempting to construct yet another appeal on his own. Zellner picked up the baton.

Meet the press: Zellner is surrounded by reporters. Photograph: Netflix

Now the lawyer is at the heart of a second season of Making a Murderer that follows her efforts to meet the high bar for overturning a murder conviction. She sets out not only to prove Avery’s innocence but to find who really killed Halbach. The courts will ultimately decide if Zellner has delivered, but if the earlier series is a measure, she is about to find both a degree of fame and scrutiny she would appear to relish.

Speaking from the couch of her office looking across Chicago’s western suburbs, Zellner is forthright, loves unpacking details and is unwavering in her faith in her client.

If the first season of Making a Murderer was more Law and Order, with its probing of the workings of the police, lawyers and the courts as they juggle Avery’s fate, the second is closer to CSI. Zellner spent 18 months re-enacting how the prosecution said the crime played out, testing the claims against forensic comparisons. But first she decided to test whether she was right in thinking Avery is innocent.

Zellner started out by using a controversial technique called brain fingerprinting, involving electro-encephalography (EEG) that claims to reveal whether particular information is stored in a subject’s brain. Although the process sounds farfetched, she brushed aside criticism from people with an opinion on polygraph testing.

“Brain fingerprinting isn’t lie detection. It simply determines whether you are storing information. So, if an elephant ran into the room and you saw it, I could test you and see if that information is stored in your brain. That’s the concept of it, which is far superior to doing lie detection on the polygraph,” she said.

Avery passed. But just as persuasive for Zellner was his willingness to participate.

“The fact that he wanted to do the testing was a huge indication to me that he was innocent. People who are guilty do not want you to do scientific testing. They balk,” she said. “I tell them, if I find out you’re guilty, I’m going to withdraw.”

That warning has its roots in her first shot at overturning a death sentence. Twenty-five years ago Zellner was a partner in a law firm doing corporate work for a health company and handling some criminal appeals for the state system on the side. The firm hired law students to help.

“There were three of them and one of them asked: ‘Why don’t we take a really complex case like a death penalty case? It would be really fun’,” she said.

Zellner thought it a good distraction from dull corporate work and the team took up the case of Larry Eyler, on death row for murdering a 15-year-old boy.

Wall of fame: certificates of achievement in Kathleen Zellner’s office. Photograph: Patrick Fraser/The Observer

Zellner and her young investigators were close to getting the conviction quashed because of attorney misconduct, when Eyler died in prison. By then Zellner knew a terrible secret. Eyler admitted to her that he had murdered at least 17 other people and wanted a deal with prosecutors to reveal the details in return for commuting his sentence to life. Prosecutors refused and the information remained hidden behind the obligations of attorney-client privilege until Eyler’s death freed Zellner to reveal it to the victims’ families.

She hesitates at the suggestion she might have freed a sociopathic serial killer, saying that there would have been another trial. But the experience shook her and she refused more cases until she was finally persuaded to defend Joseph Burrows – who really was innocent.

Burrows was awaiting execution in Illinois for murdering an 88-year-old man. Zellner got him out by persuading the state’s lead witness to admit she was the real killer.

“The truth is I’m drawn into these cases emotionally. After I got Joseph Burrows out because he was close to his execution date I never wanted to do another wrongful conviction case. But because I’ve done them successfully, I know it can be done. It keeps me going. On one hand it’s so painful to do these cases and yet the reward is so huge,” she said.

There’s an added attraction. Zellner revels in the painstaking work of recreating the crime.

“I do love investigation. I love figuring out. My father was a geologist. He travelled all over the world and my childhood was spent listening to him trying to figure out where oil was. He was responsible for the discovery of the oil in the North Sea,” she said.

Both Zellner’s parents were scientists and her siblings are all psychologists or lawyers.

Facing facts: Brendan Dassey, aged 17, in court in April 2007. Photograph: Dan Powers/AP

“I originally thought maybe I would be a prosecutor or an FBI agent wanting to catch the bad guy and so I find in these cases that I’m trying to solve them. I’m not just trying to get my client out,” she said.

That makes for good television but it also fills a need of the judicial system. American appeal courts are geared to confirm convictions. Zellner is frustrated that one court after another upholds dubious verdicts and then points to lower court rulings as justification without what she regards as proper attention to justice.

“When that prison door slams shut, the system begins to really fail because the standard is so ridiculously high to undermine confidence in the verdict,” she said. “The system is rigged against undoing convictions. It’s enormously difficult.”

To meet that standard, Zellner recreates every step of how the prosecution said Halbach’s murder occurred. She probes how much heat it takes to burn a body and the residue left. Zellner calls in an expert to examine the bones found on Avery’s property and concludes that the body was not destroyed where it was found.

A crucial piece of the prosecution’s case is a blood spatter inside the rear door of the victim’s car supposedly made when Halbach’s body was thrown into the vehicle. It’s the spatter that piqued Zellner’s interest as she watched Making a Murderer series one.

To prove the prosecution’s claims were false, she had a mannequin’s hair soaked in fake blood and swung around. No matter how they do it, Zellner’s team cannot recreate the pattern of dots found inside the car door. Flinging the substitute body about at a normal pace produces no spatter at all. When it is swung with abnormal force to make the fake blood fly from the mannequin’s hair, the resulting pattern is elongated by the speed with which the liquid hits the surface and does not match the neatly round dots found on the vehicle.

‘People who are guilty do not want you to do scientific testing’

Zellner sees the results as further evidence that blood was planted on the door and elsewhere in the car to frame Avery, including a smear near the steering wheel. But she departs from a central claim of the trial defence team that it was surreptitiously taken by someone in the police from a sample collected from Avery during an arrest years earlier, and put in the car.

The defence hung a lot of its case on this claim so when the prosecutor called an FBI expert to say it could not be the same blood because the sample from the vehicle did not contain a preservative mixed with the evidence in the vial, Avery’s lawyers were left with nowhere to go.

“The first giant failure in the case was his own defence attorneys,” said Zellner. “They locked into this theory about the blood vial that is completely false. I believe that the police did not plant the blood. The killer planted the blood and the blood did not come from the blood vial. The police locked in on Avery after it was confirmed that it was his blood on the car. So the killer duped the cops.”

Zellner thinks that once the police latched on to Avery, officers then began to plant other evidence against him. The lawyer and her forensic investigators meticulously put together a previously unheard theory for where the blood came from that she believes will go a long way toward eventually freeing Avery.

Gradually she settles on her own theory of what happened and who the real killer is. Then she discovers that the prosecution effectively hid a crucial piece of evidence in plain sight – evidence that pointed towards one of the state’s own witnesses. Perhaps it was a mistake, but it’s a very convenient one.

Zellner’s defence of Avery is constructed with awareness that it will draw considerable public scrutiny. “This has blown into such a huge, worldwide event that it has a downside,” she said. She names a judge she suspects is reluctant to rule in Avery’s favour because of criticism following the first series. Then there is the politics of the issue in a state where judges, sheriffs and prosecutors are elected. On the other hand, the lawyer thinks the attention could add to the pressure on judges further up the appeals system to give proper weight to the new evidence she has gathered. In the end, for all her doubts about the workings of the judicial system, Zellner said she is confident that Avery’s conviction will eventually be overturned.

“I am positive of it,” she said. “I know that he’s innocent.”

Making a Murderer Part 2 is on Netflix now