But Mr Johnson's family never accepted that theory, instead believing that he had been attacked and thrown off the cliff in a gay-hate killing.

Now, police are offering a $100,000 reward to finally get to the bottom of the case that has haunted Mr Johnson's family and left them demanding answers.

Mr Johnson, 27, was an American PhD student who had moved to Australia to study at the Australian National University and live with his boyfriend.

He made plans to meet his university supervisor on the morning of December 8, 1988, and left his boyfriend's family home in central Sydney and bought a ticket to Manly.

Two days later his naked body was found at the base of Manly's North Head.

An initial investigation by the Manly Local Area Command found no suggestion of foul play in Mr Johnson's death, and an inquest in 1989 determined that he had committed suicide.

However Mr Johnson's brother Steve never believed that and instead, launched his own investigation into his death.

Blue Fish Point lookout near North Head had, at the time of Mr Johnson's death, been one of five well-known gay beats in the Manly area, and Mr Johnson's family believed he was the victim of a cliff-top killer targeting gay men, or even a gay-hate gang.

The theory was based on similarities between Johnson's death and the murders of up to six men, including WIN television newsreader Ross Warren, in Sydney's eastern suburbs between 1987 and 1990.

In 2004 Coroner Jacqueline Milledge ruled that violent gangs preying on homosexuals probably hurled three of the group to their deaths at Marks Park, overlooking Bondi Beach. She also determined that gay bashings in the park were common and that similar attacks might have occurred at gay beats at Alexandria and Randwick.

Spurred by the findings, Johnson's brother Steve hired US investigative journalist Daniel Glick and retired NSW detective John McNamara to determine if the same scenario occurred on Sydney's northern beaches.

A second inquest into Mr Johnson's death in June last year overturned the verdict of suicide and an open finding was returned. The case was referred to the State Crime Command's Unsolved Homicide Team.

Detective Acting Superintendent Chris Olen, Acting Homicide Squad Commander, said detectives now had formed Strike Force Macnamir to review the case.

“At this stage, it is not known whether Scott's death is a result of suicide, misadventure or murder,” he said.

“With a lack of witnesses and physical evidence, this is a very challenging case.

"What we need is fresh information to help us solve this case and bring some closure to the Johnson family.”

Mr Johnson's sister, Rebecca, said her brother was a gentle, intelligent and loving man.

“He came to Australia to live with his partner and study mathematics, and died on the day he should have been celebrating completion of his doctoral work. We lost a brilliant intellect and a beautiful person at much too young an age," she said.

Steve Johnson said he remained convinced that his brother was targeted in a gay-hate crime.

“We understand that police need to look at all the different scenarios, and support them in that effort," he said.

"But we believe in our hearts that Scott was murdered in a hate crime because he was gay. Today we are pleading for people to come forward with any information they have about Scott's death.”

Anyone with information about Mr Johnson's death has been urged to contact Crime Stoppers on 1800 333 000.