A quarry will be drained of water to allow police to continue a murder investigation into the disappearance of a mother and her three-year-old son more than 40 years ago.

Renee MacRae, a 36-year-old mother-of-two, and her son Andrew were last seen driving out of Inverness on 12 November 1976. Her BMW was found burned out in a layby on the A9 near Dalmagarry, 12 miles south of the city, and the pair have not been seen since.

A forensic search of Leanach quarry is expected to start in the coming days, led by Police Scotland’s major investigation team, in the latest attempt to move the investigation forward.

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Det Insp Brian Geddes, the senior investigating officer, said: “Work is now under way to carry out a forensic search of Leanach quarry, where we believe key pieces of evidence may have been concealed.

“A key part of this operation is to recover the remains of Renee and Andrew if they are within the quarry and gather evidence vital to this ongoing investigation. We are working with a number of different partners on what will be a challenging and complex operation expected to last several weeks. I would also like to thank local landowners for their cooperation in this inquiry.

Facebook Twitter Pinterest A police forensics team move earth at Dalmagarry quarry in search of Renee MacRae and her son Andrew. Photograph: Maurice McDonald/PA

“The quarry has previously been deemed too dangerous to search using divers, which is why the decision has been made to pump the water completely from the site. We will thereafter remove the sediment from the quarry, which will allow specialist search officers and forensic experts to carry out a meticulous search of the material.”

A full cold case review resulted in a month-long excavation of Dalmagarry quarry in 2004.

Several days were spent at Leanach quarry in October last year when officers also released an image of young Andrew around the time of what would be his 45th birthday. A picture of his pram, which was never recovered, was also released.