SYDNEY, AUSTRALIA—On a December day in 1988, a teenager on a spearfishing expedition found a body at the bottom of one of the wild, honey-coloured sandstone cliffs that line Sydney Harbour.

Naked, torn and battered by the rocks, the dead man was a promising U.S. mathematician, Scott Johnson. His clothes were found at the top of the cliff in a neat pile with his digital watch, student ID and a $10 bill, folded in a small plastic sheath. There was no wallet, and no note.

The police concluded that Johnson, 27, had committed suicide, and a coroner agreed. Fatal leaps from the cliffs around Sydney into the fierce sea below were not uncommon, then or now.

But 28 years later, a new inquest into Johnson’s death has begun. His brother, a wealthy Boston tech entrepreneur, has pressed the Australian authorities for years to revisit the case, arguing that Johnson was murdered because he was gay and that the police failed to see it.

If so, it appears Johnson may not have been the only one.

During the 1980s and 1990s, the Australian authorities now say, gangs of teenagers in Sydney hunted gay men for sport, sometimes forcing them off the cliffs to their deaths. But the police, many of whom had a reputation for hostility toward gay men, often carried out perfunctory investigations that overlooked the possibility of homicide, former officials and police officers say.

Now the police in New South Wales, the state that includes Sydney, are reviewing the deaths of 88 men between 1976 and 2000 to determine whether they should be classified as anti-gay hate crimes.

About 30 of the cases remain unsolved, and the police have not said how many of the killings were tied to gangs. About a dozen victims were found dead at the bottom of cliffs or in the sea, the police say.

The review and the inquest into Johnson’s death are casting light on a shocking chapter of Sydney’s history, one that some say has yet to be fully revealed.

No new arrests have been made in connection with the killings since the review began in 2013, and the police declined to discuss the open investigations. In many of the cases, the police said, relevant evidence had not been collected at the time or has since been lost.

“While the review is a difficult task because we can’t rewrite history, we know it is important we do everything we can to ensure the best outcomes in the future,” said Tony Crandell, an acting assistant commissioner for the New South Wales Police Force.

But others have suggested that the review, which aims to determine which cases may involve bias but not to solve them, is not a sufficient response.

Researchers who have studied the matter say the gangs were loose alliances of young men, teenage boys and sometimes girls who looked for victims to harass and assault at Sydney’s so-called gay beats — places where gay men were known to meet, including secluded spots on the cliffs. The gang members called it “poofter bashing.”

Few victims would have gone to the police, Tomsen said. Most gay men were closeted, and many would have feared being assaulted by the police themselves. After the city’s first gay Mardi Gras parade was broken up by the police in 1978, some marchers were beaten in their jail cells.

“Any gay who was attacked would be seen as a foolish risk-taker if they reported that attack to police,” Tomsen said.

Still, there were some arrests and prosecutions. In 1990, a Thai man was attacked with a hammer at the top of a cliff and fell off the edge. Three teenagers were arrested and convicted of murder.

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The idea that the killing was part of a pattern was not seriously pursued until years later. In 2000, Page, spurred by letters from a grieving mother, reopened the case of Ross Warren, a 25-year-old television news anchor who disappeared in 1989.

Warren’s body was never found, though his car keys were discovered in a rock ledge. The police concluded that he had accidentally fallen into the harbour. But Stephen Page, a former New South Wales detective who reopened some of the cases years later, found the original investigation had been cursory at best.

“There was no crime scene, no evidence, and no witnesses to Ross Warren’s disappearance,” he said.

Page began looking into similar cases. In 2005, an inquest concluded that Warren had been murdered, another man had been pushed or thrown from a cliff, and there was a strong possibility that a third man had been, too.

“This was a grossly inadequate and shameful investigation,” Magistrate Jacqueline Milledge, a deputy state coroner, said of the police handling of Warren’s death.

When Steve Johnson learned that such cases were being revisited in Sydney, he felt he finally had a possible explanation for his younger brother’s death. Johnson had looked out for Scott since childhood, when their parents divorced, and he considered suicide impossible.

“This was my brother, the person I was closest to, my soul mate,” Johnson, 57, said in December, outside the Sydney courtroom where the inquest began.

In 2012, a new inquest overturned the original finding of suicide. But the coroner reached no conclusion about how Johnson had died, saying that while anti-gay violence was a possibility, so was an accidental fall.

When the current inquest resumes in June, it will hear new evidence, the coroner’s office has said.

Whatever the result, Steve Johnson and others hope it will spur further investigations of these cases.

“There was clearly a pattern to these deaths,” said Margaret Sheil, whose brother Peter was found dead at the base of a cliff in 1983. “Today, it is extraordinary to think that we would not have had an open discussion about what happened. And if we had, it might have prevented it happening to someone else.”