A criminologist who spent a year investigating the murder of Bronwynne Richardson does not believe Colin Michael Newey was involved.
The other man previously charged was Bronwynne's ex-boyfriend Geoffrey Charles Brown, who a 2011 inquest heard had been acting strangely and made threats after the break up.
In both cases, charges were dropped. No one has been convicted.
University of Newcastle's Xanthe Mallett looked into the cold murder case of Bronwynne as a part of the Nine network television series Murder, Lies and Alibis.
Dr Mallett told The Border Mail she examined minute changes in Mr Newey's body language, including eye and hand movements, while he was being questioned about the murder.
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She concluded Mr Newey was likely not involved in his second cousin's murder before he sat a polygraph test but noted he was uncomfortable talking about her.
"I don't think Colin Newey was there judging by what I saw of him and how he answered questions," she said.
Dr Mallett also said she believed Mr Newey did not call Murray Bridge police claiming to have been present at the time of the murder.
In Murder, Lies and Alibis, Mr Newey was presented as one of two men who could have made the call - the other was Ross Eames.
Dr Mallett said she has a theory about what transpired the night Bronwynne died, but can't say too much because she doesn't want to compromise the police investigation or jeopardise any future court trial.
"We do believe a number of people know what happened, some of who are still alive," she said.
Despite decades passing, Dr Mallett believes Bronwynne's murder will be solved.
"In these really old, old cases you need someone to break it open," she said.
"We've got a lot of information about where she was and where all those individuals were but there's something missing. The crucial eye witness is missing."