The person who knew the answer died with his secret.
But police may be getting closer to solving the mystery of where William MacDowell buried his victims, Renee and Andrew MacRae.
The double murderer was jailed a year ago for killing his lover and their son in 1976.
Renee and Andrew disappeared in November that year.
Her BMW car was later found alight in a lay-by at Dalmagarry on the A9, some 10 miles from Inverness.
MacDowell died five months after the conviction without revealing where the bodies are and before detectives could question him further.
Now police are name-checking four areas around Inverness as possible sites of interest and looking for a derelict cottage.
But how do these areas fit in with what we know so far about the key sites in the case?
Renee and Andrew: Why is there a new appeal?
There is still “unfinished business” in the two-fold investigation to catch the killer and recover the bodies.
The case is back in the public eye with a two-part documentary on the 46-year-old investigation and trial first airing last week.
It has prompted new calls to the police team and now a new appeal for help.
Ahead of the programme airing, Detective Chief Inspector Brian Geddes, who led a re-investigation of the case, revealed historic information was being reviewed.
A member of the investigation team continues to examine evidence and look for other possible locations to search.
Police received a number of calls during the trial and afterwards suggesting possible sites to look for the bodies.
At the time, just a few weeks ago, DCI Geddes said no new information had been provided and no new searches were warranted.
He did say all data was logged and added to the review.
But something has changed in the past few weeks, and now it looks like a new appeal and search are worthwhile.
Police have even issued a list of areas they that could be relevant to their hunt.
Which sites are involved in new search for Renee and Andrew MacRae?
These include sites where MacDowell and Renee may have met during their relationship, including the Blackfold area above Loch Ness, Inverfarigaig, and between Nairnside and Daviot.
In addition, the review includes properties possibly linked to MacDowell which he could have used, including those now empty or derelict.
In particular, officers are trying to find the location of a cottage which could hold the vital clues.
Years before MacDowell was convicted, he told a witness that if the building was found, it would lead to his downfall, police have revealed.
Despite extensive enquiries, they have failed to trace it and hope the latest public appeal may help.
Take a look at how the new sites sit on a map with the key places that we already know about.
The team is also considering the potential to use existing and emerging technologies including drones to identify potential burial sites.
Case not closed until bodies are found
The renewed effort is aimed at completing the second part of the dual investigation.
“This case will never be considered resolved until we find Renee and Andrew”, says DCI Geddes.
“We owe it to the family and friends to make every effort to recover Renee and Andrew’s bodies.
“I would urge anyone who may be able to assist, no matter how small you may feel your information is, to come forward.”
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