Three other statements obtained by Mr Bruce reveal that the young mother was so frightened that she would not stay home by herself and spent the day with Harvey Crewe on the farm.
Bruce told the Herald the letters and statements added weight to his belief that a possible third suspect was not properly investigated by the police.
Local farmer Arthur Allan Thomas was convicted of the killings at two trials but was pardoned in 1979, leading to speculation that Jeannette Crewe's father, Len Demler, was the prime suspect.
"I went back through all three key pieces of evidence. The three key pieces are the gun, the wire and the axle stubs," said Bruce. "Having done all that, I'm persuaded that Arthur Allan Thomas is innocent. I'm also persuaded that Len Demler is completely innocent. And I have uncovered material that points towards a third person theory."
His documentary would explain the evidence which suggests a third person had access to a rifle, one of two which the Crown's original firearms expert could not exclude as the murder weapon.
Bruce said his research suggested that Jeannette Crewe was a "grounded, solid person" whose fears about the suspicious fires and unsolved burglary seem to have been dismissed by Detective Inspector Bruce Hutton, the head of the homicide inquiry.
He did not believe Mr Demler was responsible for the burglary or fires, as the Crewes went to live with him after the fire which damaged their home.
Bruce also backed Rochelle 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" and supported her call for an independent inquiry.
* The Investigator Special: Who killed the Crewes? Screens on TV One at 8.30 tonight.