HE is the experienced detective who took just three weeks to decide that Margaret Fleming’s carers were actually her killers.

But the man who led the investigation into the disappearance, and later murder, of vulnerable teenager Margaret Fleming will always be “troubled” by not knowing exactly what happened to her.

The case crossed Detective Superintendent Paul Livingstone’s desk on Monday, October 31, 2016, four days after Ms Fleming was reported missing by Avril Cairney and Edward Jones following concerns being raised over a benefits claim made by Jones in the missing woman’s name.

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As head of the CID and the Major Investigations Team, Det Supt Livingstone was drafted in by the local division when more manpower was needed.

He said: “At that point she was very much a missing person but a few things were not sitting right.

"There were a number of discrepancies, which is not unusual, but we had a high-risk, vulnerable young female missing that was going to take a lot of work [to find her] if everything we had been told was true.”

The last time Ms Fleming, who had learning disabilities, was seen by anyone other than Cairney and Jones was on December 17, 1999 by Jones’ brother.

The couple claimed that she had run off to London, only returning to the house, Seacroft on Inverkip, Inverclyde, when she needed money.

It was Det Supt Livingstone’s “professional intuition” that left him suspecting that Ms Fleming’s fate was a grisly one but he had to rule out every other possibility and gather enough evidence to be able to detain, and later convict the couple.

The lack of evidence told its own story to Det Supt Livingstone. When a sniffer dog identified an area of the garden where there were “indications of human decomposition”, Cairney and Jones became murder suspects with police believing Ms Fleming to have been killed between December 1999 and January 2000.

He said: “Three weeks into the inquiry was the day I decided [Cairney and Jones] were suspects. I knew if we did our job right, we’d get them for this. I always had that confidence and faith.”

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Operation Haerford involved more than 200 officers, forensic experts and other police personnel in a case that spanned almost three years and crossed the Atlantic Ocean. The team had to glean the facts despite there being no physical or visual trace of Margaret for more than 16 years.

Forensic experts excavated the garden but no body was found. Only fragments of bone were discovered, but were so badly charred they couldn’t be identified as human or animal.

Officers spoke to Ms Fleming’s family, some as far away as America, with every piece of information used to knit together a timeline of her life since the day she was born to the day police started their inquiries.

Every single doctor’s and hospital appointment, or contact with social care she had ever had was documented – but nothing from 1999 onwards.

Det Supt Livingstone said: “There’s absolutely no way someone, particularly with her vulnerabilities, could go 16 to 18 years without seeing a doctor.”

More than 1,000 statements were taken from neighbours past and present, postmen, cold callers, bus and taxi drivers, gas and electric engineers, food delivery drivers and the friends and family of Avril Jones, who Margaret had spent time with – anyone who might have encountered her from when she moved into the house in 1996 to when she was last seen three years later.

Police swept the house, “stripping it” and ripping up floorboards to find evidence of Ms Fleming having lived – or died – there, but they came up with nothing. A teddy bear, pair of socks and t-shirt Jones claimed belonged to her contained not a trace of her DNA.

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CCTV footage was pored over, records from James Watt College where she briefly attended, were pulled, and every potential sighting fully investigated.

Trying to prove a negative was challenging for the Major Investigations Team, but they didn’t give up, believing themselves to be advocates for missing Margaret.

“They kept taking statements and we started to unearth the neglect, the violence and abuse [Ms Fleming suffered].”

Ms Fleming was starved and bound by Jones and Cairney. Her arms were threaded through plastic tubes and her hair chopped short.

She was kept isolated when the couple had visitors and one witness reported seeing her with her hands taped up, causing her circulation to be cut off.

After exhausting their “proof of life” investigation, there was enough evidence to detain Cairney and Jones.

Det Supt Livingstone said: “When you’re trying to prove she’s alive and you can’t, then she’s got to be dead. We could back it up that they were the only two people that had the opportunity.

"We had to paint that picture to show in all the circumstances there’s only one outcome here and that’s that they’ve killed her.”

It was a difficult case for Det Supt Livingstone, who was on the scene at the Clutha disaster and at Glasgow’s George Square bin lorry crash.

He said: “They’ve been denying it for nearly 20 years now. Is there anything left of Ms Fleming to find? Only they know. I would personally like to know, if I don’t, that’s something I will take with me to the end of my days.

"Because we don’t have the answers, that will always trouble me. We’ve got justice but not the answers.”