Former Australian officials have said the police at that time were often hostile toward gay men and complacent about investigating their deaths. In 2013, the police of New South Wales, which includes Sydney, opened a review of 88 deaths of men between 1976 and 2000 to determine whether they should be reclassified as hate crimes.

Image Scott Johnson in 1987. He was pursuing his doctorate in Australia when he was killed. Credit... Steve Johnson

No new arrests have been made since then, and Magistrate Barnes said there was insufficient evidence to determine who killed Mr. Johnson.

Mr. Johnson’s brother, Steve Johnson, an American tech entrepreneur who hired an investigator and pushed for the case to be reopened, said Thursday that he never believed his brother had committed suicide. When he learned about a 2005 inquest into three deaths from the same period, which found that the men had been driven off cliffs, “everything clicked into place,” said Mr. Johnson, 58, who was in court for the Thursday ruling.

On Thursday, Magistrate Barnes said the initial investigators in Scott Johnson’s case had “quickly jumped to conclusions without thoroughly and impartially examining all the facts.”

Magistrate Barnes said that the cliff from which Mr. Johnson fell was known to many as a so-called gay beat, a place where gay men would meet up. Former Australian officials now acknowledge that groups of teenagers and young men would target such areas with the intention of assaulting gay men, and that the police failed to see it at the time.