Scott Johnson's death was ruled a suicide. Decades later, his brother's tenacity led to the truth.

Nearly 35 years ago, Scott Johnson, a gay American mathematician, was found dead beneath a cliff in Sydney, Australia. Investigators quickly determined the 27-year-old died by suicide.

But his older brother, Steve, had his doubts -- how could his brother, a brilliant PhD student with a bright future in mathematics, take his own life? Or was something more sinister at play?

Those unanswered questions set Steve Johnson on a dogged, multi-decade quest for justice, and he wouldn't stop seeking answers until uncovering the astonishing truth just within the last several years.

"Never Let Him Go," a new four-part documentary series produced by ABC News Studios, Show of Force and Blackfella Films for Hulu, delves into Scott's extraordinary life and mysterious death, along with Steve's relentless efforts to piece together what really happened to his brother during a time when a wave of anti-gay violence was plaguing the Sydney area.

"Can't look at that drop without thinking of Scott and the terror he went through for the couple of seconds that it took him to fall. Did he trip? Was he pushed? Did he kill himself? You know, I had a blizzard of questions," Steve says in the series.

After learning of his brother's death in December 1988, Steve says he left the U.S. for Canberra, Australia, where Scott was living with his partner at the time. From Canberra, Steve says he drove around three hours away to Manly, the town just outside of Sydney where Scott died, to talk to Troy Hardie, the police officer who investigated the case.

"My initial finding of suicide was done in good faith," Hardie says in the series. "And was made on all the evidence that was presented at the scene, or lack of evidence that was presented at the scene. [Scott] was found naked at the bottom of the headland, with his clothes neatly folded at the top of the cliff with his identity clearly displayed on top of those clothes when I first viewed them."

Hardie also said he spoke to Scott's partner, who told him that Scott had previously thought about suicide.

"And the time when I handed it off to the detectives, there was no evidence there to indicate anything other than suicide. And that was what I put in my report of death to the coroner," Hardie said.

At the time, Steve recalls feeling "mystified."

"It just didn't seem possible that he would have killed himself without saying goodbye. I couldn't imagine. He had has little niece who was 6 weeks old. He would have wanted to see his niece. And the constable just listened to me, expressed his sympathy and waited for me to leave," Steve said.

With a new baby at home, Steve Johnson flew back to the U.S.

"But even on my trip back home, and even during the funeral we had in California, I was far from letting go of this," Steve said.

The unanswered questions continued to gnaw at Steve in the weeks that followed. He says he spent the rest of December typing up a 50-page document to send to the Manly police with questions he thought they should investigate. Through a friend who worked in city politics, Steve says he got Sen. Ted Kennedy to write a letter to the ambassador in Australia, inquiring about Scott's death.

The following year, there was an inquest conducted by the coroner into Scott's death. To Steve's frustration, it reaffirmed the initial finding that he died by suicide.

"And the coroner said that 'I'm sorry to say, Mr. Johnson,' he stared right at me, 'that your brother was introverted, and intelligent. And unfortunately, those are just the type of people that tend to commit suicide. Your brother committed suicide.' Gavel to close, and that was the last official act on Scott's case for 20 more years," Steve said.

"The police told us that it was suicide, and that was the truth, like, the capital T, Truth," Scott's sister, Becca Johnson, says in the series. "And that left us, as a family, to figure out, like, how did we miss that Scott could kill himself?"

For years, the tragedy hung over the family as Steve Johnson raised three children with his wife Rosemarie and advancing his burgeoning career in tech.

Things were quiet for 17 years. Then Scott's partner sent an article to Steve.

"We learned about these men are showing up [dead] at the bottom of cliffs. And everything started again," Rosemarie said.

In 2005, the family learned that police were reopening their investigations into the unsolved deaths of three gay men who were also found dead at the bottom of cliffs around Sydney in the 1980s.

As Steve learned more about his brother – and his possible murder – he refused to let go, enlisting the help of investigative journalist Dan Glick, taking his story to the media and making enemies along the way.

His persistence paid off. Scott's case became internationally known and eventually sparked a public reckoning over Australia's history of brutal and ingrained homophobia.

"Never Let Him Go" weaves together interviews with Scott's family and friends, investigators who worked the case, and members of Sydney's gay community. It features home video, rare footage of Sydney in the 1980s, and never-before-seen evidence from the investigations into Scott's death.

The first-person narratives combined with the visuals tell an intimate story of Scott's life and death, and the evolution of Sydney's gay community.

"I let go of even the remotest chance that Scott had suicided," Steve said. "It was quite a windfall of relief for me in some sense, although suicide's just another mystery. So, I was trading that mystery for this one. Who killed him? Why?"

"Never Let Him Go" begins streaming Sept. 6 on Hulu.

If you are struggling with thoughts of suicide or worried about a friend or loved one, call or text the Suicide & Crisis Lifeline at 988 for free, confidential emotional support 24 hours a day, seven days a week.