A new television documentary examining New Zealand's most famous murder case supports Rochelle Crewe's belief that her grandfather is innocent of murdering her parents - and nominates a third suspect who was not properly investigated.
But a full copy of the internal police review into the unsolved murders of Jeannette and Harvey Crewe to "provide answers" to their only child may not be released to her.
Local farmer Arthur Allan Thomas was convicted of the June 1970 killings at two trials, but was pardoned in 1979, leading to speculation that Jeannette Crewe's father, Len Demler, was the prime suspect.
Despite the damning findings of a royal commission in 1980, police did not actively re-investigate the case, which Rochelle Crewe - who was 18 months old when her parents died - said has "led to harmful speculation about my family".
Bryan Bruce, who has analysed some of New Zealand's best-known crimes in books and a television series, has turned his attention to the Crewe murders.
While reluctant to divulge details of his research before The Investigator: Who killed the Crewes? screens tomorrow night, he confirmed he believed Mr Demler was unfairly tarred by rumours after Mr Thomas was pardoned.
"The thrust of my documentary is that Len Demler didn't do it. And neither did Arthur Allan Thomas," he said.
"There was a third person, a third option, that wasn't properly investigated, in my opinion."
Mr Bruce also backed Ms Crewe's concerns over the internal police review which was announced after she broke her 40-year public silence in the Herald in October 2010.
He said the police were "looking into their own behaviour reluctantly and only because she had shamed them into it".
Ms Crewe asked the police to reopen the case, and when they refused, she sought an independent inquiry by an overseas judge into their handling of the case.
That request was also denied. Instead, Detective Superintendent Andy Lovelock was appointed to review the original homicide inquiry, and subsequent evidence which was uncovered with modern police tools.
When the review was announced, then-Deputy Commissioner Rob Pope said it was to "provide answers" for Ms Crewe.
While the review is not finished, Ms Crewe said police had denied her full access to her parents' homicide file, and she might not receive a copy of the completed review.
"I believe that as a family member I should be entitled to see the whole file so I know all has been done that can be possibly done," she said. "Why spend money on a review if only the police themselves get to see it?"
Mr Lovelock is overseas, but Assistant Commissioner Allan Boreham said the police had committed significant resources and expertise to the review of the Crewe murders.
He said Commissioner Peter Marshall had ordered that an independent senior barrister review the file when it was finished.
But Mr Boreham stopped short of saying the review would be given to Ms Crewe or made public.
"We know there is a very strong and enduring public interest in this case, which has been the subject of many court processes, a royal commission of inquiry and numerous books and articles.
"It is not appropriate for us to be partially releasing any further information into the public domain at this stage, nor can we speculate on any decisions which may be made about release of information."
The Investigator Special: Who Killed the Crewes?
Screens on TV One at 8.30pm tomorrow.