Renee MacRae, 36, and her three-year-old son Andrew were murdered by William MacDowell after they disappeared from their home in Inverness in 1976.
MacDowell evaded justice for decades but was found guilty of their murders in 2022 and handed a life sentence with at least 30 years behind bars.
Married MacDowell, who had been having an affair with Mrs MacRae, died in February 2023 at the age of 81.
He went to his grave without revealing where he had disposed of the bodies, but police are currently examining a potential breakthrough in the case.
A Police Scotland spokesperson said: “On Thursday 13 March 2025, officers attended the A9 near Dalmagarry after an item was recovered.
“The item is being examined and further enquiries are ongoing.”
Mrs MacRae and Andrew have been missing since 12 November 1976.
The pair left their home in the Scottish Highlands, with Mrs MacRae’s BMW car found later that day on fire in a layby on the A9 near Dalmagarry.
MacDowell and mother-of-two Mrs MacRae had been having an affair for more than four years when she vanished.
Mrs MacRae was estranged from her husband, Gordon, who was MacDowell’s boss at Hugh MacRae Builders Limited.
MacDowell denied any wrongdoing and was brought to court each day in a wheelchair by his wife Rosemary.
However, jurors rejected his claim the murders were committed by somebody else and convicted him of the crimes.
Judge Lord Armstrong described the murders as “executions”.
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Investigations have continued since 1976, with Dalmagarry Quarry searched again during a month-long excavation in 2004.
In 2019, Leanach Quarry was excavated as part of the probe.
And in August 2023, police said officers were looking into properties linked to MacDowell as well as other places he may have met Mrs MacRae during their relationship.
Detective Chief Inspector Brian Geddes – who helped bring MacDowell to justice – said at the time: “This case will never be considered resolved until we find Renee and Andrew.”
