Forensics officers have started sifting through decades of sediment from a quarry emptied in the search for Renee MacRae.
Police began draining Leanach Quarry earlier this month, with forensics officers drafted in from Inverness, Aberdeen and Dundee to examine its contents.
A car has been pulled out of the water, but the detective leading the investigation ruled out its relevance to the 43-year-old case.
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He revealed that while the forensics teams get to work, his officers will be re-interviewing 125 witnesses from across the Highlands.
Mrs MacRae and her three-year-old son, Andrew, vanished on November 12, 1976.
Her burned-out BMW was later discovered in a lay-by on the A9 south of Inverness – 12 miles from the quarry.
Detective Inspector Brian Geddes remained tight-lipped about what they were looking for in the quarry, but said it was the “standout location” for evidence based on information received.
Over the last 10 days, 16 officers have removed nearly 13 million litres of water – pulling out a car, old tyres and steel drums.
DI Geddes said: “There are undoubtedly challenges ahead with the capacity of the quarry, but we remain really determined – and I remain confident – that if there is evidence in there then we will find it.
“I’m afraid I can’t say anything further. It really was a culmination of a review of everything we have gathered to this stage from 1976 and now our current view of that which has led us to believe that of all the locations mentioned, this is the key one for us.
“Nobody could tell us exactly what we would face until we got the water out.
“Now that we have, it looks a bit daunting to pretty much systematically search all of that but it is what we are geared up for and it is what we are going to do. We have committed to it and we will see it through.
“Rest assured for the friends and family of Renee and Andrew, we won’t be leaving here until every inch has been searched.”
Teams are trying to establish the depth of the silt so they can establish a timescale for the thorough analysis of materials, and to also establish the total outlay of the project.
There are “hundreds of tonnes” of silt for the team to go through.
Divers previously searched the quarry years ago and found evidence of about 20 cars under the water, but DI Geddes does not believe they will be relevant to the investigation.
He added: “There’s the one car come out so far and there is a trailer on the banks there just now. It’s what we expected. From the dive footage in 1977 you could clearly see vehicles in the water and there was an estimate of 20-30 vehicles within this north pond alone.
“We are not seeking any vehicles in relation to the case. Any vehicles in there will be removed and searched for what may be inside them, but there are no vehicles relative to the case that we are looking to find.”
On November 12 1976, Mrs MacRae left her home in Inverness with three-year-old Andrew, heading south on the A9.
Mrs MacRae, who was estranged from her husband Gordon, had set off for Perth to meet her lover, Bill McDowell, Mr MacRae’s married accountant, however, he said they never met. She has not been seen since.