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As the tenacious lawyer battling for justice in Netflix’s Making A Murderer, Kathleen Zellner has won an army of fans for her never-say-die spirit.

What the viewers don’t know is that it comes from the man who discovered oil in the North Sea – Kathleen’s dad.

The 61-year-old has righted more wrongful convictions than any other attorney in the US. Her latest cause – to free Steven Avery from a sentence for a murder she believes he didn’t commit – is the subject of the acclaimed documentary.

Zellner revealed her meticulous evidence-gathering skills and dogged determination to get to the truth come from her geologist father Owen Thomas, who spent years trying to extract the black gold he discovered off the Scottish coast.

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She said: “Persistence was the trait my father valued over any other. Many times, as I was growing up, he would be told that an effort to drill for oil in a particular location or with certain equipment had failed.

“He would never accept defeat and he would always try another approach – dig deeper, use another drill, change the angle.

“I will succeed in freeing Steven Avery. His case has already opened the eyes of millions of people as to the abuses, corruption and biases of the American justice system, which is replicated one way or another all over the world.

“If I free Steven Avery, it’ll benefit thousands of other incarcerated innocent individuals all over the world. Hopefully my methodology, use of scientific evidence and focus on uncovering the truth will change the way wrongful conviction cases are approached.”

Zellner, whose fierce intellect and southern drawl have captivated viewers, wants to overturn the conviction of Avery for the murder of photographer Theresa Halbach.

Her approach echoes her father’s battles to convince his bosses at Phillips Petroleum that the seismic data they had did point to a massive oil field in the North Sea. Her dad’s discovery of the Ekofisk oil field helped to transform Scotland, the UK and Norway into major oil-producing countries.

Avery, 56, was given a life sentence for the murder of Halbach in 2007, without the possibility of parole, by a Wisconsin jury.

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His nephew Brendan Dassey, 29, was also given a life sentence, with the earliest possibility of parole in 2048.

Making A Murderer Season 2 picks up 10 years after they were convicted, with Zellner now acting as Avery’s attorney.

The case has helped Zellner to acquire more than half a million Twitter followers.

The Chicago-based lawyer funds her work on wrongful convictions through other work at her practice.

Zellner says she is utterly convinced of Avery’s innocence and has developed her own theory on Halbach’s killer.

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Her main theory centres on Dassey’s brother Bobby being a potential suspect.

She details how Dassey’s stepfather Scott Tadych visited Bobby at the family business on the day Halbach died.

And with a recent series of explosive tweets, she refers to the person who she links to the murder as the “suspect”.

Zellner said: “Having succeeded in exonerating many innocent people, I know that, in the face of setbacks, the key to success will be to persevere, to try another approach, to work harder but, above all else, as my father taught me, never ever give up.”

Zellner, who can trace some family roots back to Scotland, has a daughter, Anne, who is also a lawyer.

While her father instilled an analytical approach to problem solving, it was her mother Winifred, a former nurse, who taught her to be empathetic and to listen very carefully.

She said: “My mother believed everyone’s behaviour made sense if you understood the important events in his or her life.”

Zellner’s father also served as a soldier and was commander of the 831st Engineering Battalion in World War II. He saw combat in Europe for five years.

She said: “His battalion were responsible for rebuilding and repairing bombed-out bridges and roads so the Allied troops could move forward. He greatly valued grace under pressure and thought the Scottish, English and Welsh embodied this trait.”

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When he returned to his geology and engineering work, his discoveries marked the first commercial oil find in the North Sea and the first giant field in Western Europe.

It was a turning point for the industry. Reports claim that the Phillips group already had 32 holes on the Norwegian shelf with disappointing results and had ordered that drilling be farmed out. But Thomas and his team were confident in their data and persevered.

The discovery of Ekofisk in 1970 helped mark an end to Scotland’s oil imports. It also transformed Norway’s fortunes and turned sleepy farming villages into a lucrative industrial hub.

Zellner says her father was “very proud” of his contribution to the discovery of the Ekofisk oil field and the many other fields he discovered all over the world.

She said: “My father loved geology and saw it as a means of understanding the history and evolution of the world.

“Also, he was an engineer so he was able to not only discover oil fields but to devise a way to extract oil from the ground or water, wherever its location.”

No doubt, he would also be very proud of his daughter’s success.

For Zellner, success will be measured in freeing Avery, which would amount to her “best accomplishment”.

She said: “I will have carried on my father’s legacy of making the world a better place, in some small way, because I existed for a while in this very difficult, challenging world.”