WHEN the naked and crumpled body of US maths genius Scott Johnson was found off Sydney's North Head, police dismissed his death as suicide.

Nearly 20 years later, his family has launched its own investigation into the possibility the PhD student was the victim of a cliff-top killer targeting gay men, or even a gay-hate gang.

The extraordinary theory is 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 has hired US investigative journalist Daniel Glick and retired NSW detective John McNamara to determine if the same scenario occurred on Sydney's northern beaches.