An elite police criminal profiler built a psychological model of the murderer who killed the Crewes - and ruled out the motive detectives say drove Arthur Allan Thomas to commit the crime.
The profiler also diminished the role of Jeannette Crewe in any motive, saying a dispute between her husband Harvey and the killer likely festered for four years until the murder was committed through a "distorted desire to punish".
The expert at the police's Criminal Profiling Unit studied the police file of the 1970 cold case murders used in a landmark review and came to several conclusions about the killer.
The unit is one of the police's secret and specialist teams and its expertise was built into the review into the Crewe murders, released this week after their daughter Rochelle asked Prime Minister John Key for answers to the cold case mystery.
The style of crime-solving uses techniques made famous through TV crime solvers such as "Fitz" in the popular British show Cracker, or the Criminal Minds team.
They use behavioural science, criminology and psychology to provide breakthroughs and leads to bolster policing techniques.
The profiler identified up to six criminal acts against the Crewes in the four years before their murders. They included a burglary in 1967, a fire in the Crewes' nursery in 1968 and the destruction by fire of a hay barn in 1969 - the year before the double murder.
The pattern showed "escalating criminal progression".
"Someone did not like them and their hatred was evolving over time. The burglary and fires were precursor offences by the perpetrator of the murders."
The criminal profile said the killer knew the Crewes well and "wanted to intimidate" and "possibly drive them out of the district".
Harvey Crewe had a "dual personality" with a quick temper. He may have had a mood disorder, the report stated, and people reported being "very frustrated and upset" after dealing with him.
"In my opinion, one of these people went on to commit the murders. The original issue may have been minor but this individual felt aggrieved by whatever Harvey had done. As a result he began to brood and ruminate."
When Harvey Crewe arrived home to find the house on fire in 1968 and extinguished the blaze, the failure to destroy it completely "probably fuelled the perpetrator's hostility and resentment".
A lack of reaction after burning down the haybarn further angered the arsonist who then turned to murder.
The killer would have had easy access to the Thomas farm, knew the rifle was left in a variety of places and had gone there to take it.
The night of the murders, the killer "executed" Mr Crewe from behind with a shot to the head because "ultimately he was still too scared to confront Harvey directly". Jeannette Crewe's death followed because she had either seen the killer or could work out who it was, the profiler said.
"Rochelle Crewe was ignored because the perpetrator's primary grievance was with her father."
After the murders, the bodies were dumped in the river to compromise the police investigation.
The killer would become cautious of officialdom and stop many normal activities, the profiler said.
The stress of the double murder would have had a "corrosive" effect on his behaviour. Were there a relationship it would have failed and there would have been employment issues and possible substance abuse.
The profiler also ruled out the "infatuation" motive which police claimed led Mr Thomas to murder the Crewes. The finding is significant because evidence which secured the murder convictions against him was underwritten by a motive of spurned love turned to homicidal rage.
The profiler also said circumstantial evidence - including three items linked by the review to the Thomas farm - was "quite damning".
However, he said after reviewing the police file "I am not convinced Arthur Allan Thomas murdered Jeannette and Harvey Crewe".