DIRT ingrained in Alexander Pacteau's tyres provided police with the vital link between the killer and his victim.

The patches of soil, which were seized from the wheels of his Ford car, proved decisive in securing Pacteau's admission of guilt.

Soil expert Professor Lorna Dawson, who was involved in the World’s End murders case, was enlisted by police and prosecutors to pinpoint the origins of the soil.

The samples were small - but it was enough to link Pacteau to where he had hidden Karen Buckley's remains and dumped her handbag.

Ms Dawson, a geoforensics expert from the Hutton Institute in Aberdeen, pieced together the damning evidence, and the samples taken from the tyres matched samples from two locations linked to the murder.

From her analysis, Ms Dawson concluded that the car had been at both Dawsholm Park and High Craigton Farm in the days and hours after Karen's disappearance.

It was the breakthrough police had been waiting for, but detectives continued to work for days, in secrecy, to make their case against Pacteau watertight.

Fortunately, the soil was not the only piece of scientific evidence linking Karen's brutal killing to 21-year-old Pacteau.

While searches and tests were carried out on his vehicle, a specialist police sniffer dog indicated that a dead body had been the boot of the car.

Droplets of blood were also found near the passenger seat inside the vehicle, despite a "full valet service" being carried out on Monday afternoon.

Karen's friends gave police a toothbrush which belonged to the nurse and the sample taken from this soon proved to be a DNA match for the blood in the car.

Armed with this mountain of overwhelming evidence, detectives detained Pacteau.

Sources close to the investigation told the Evening Times how his capture was an "hugely satisfying" moment.

They said: "We had known he had done something to Karen, there was no doubt in our minds about that.

"But, we had to bide our time and make sure we had as much evidence as possible, before bringing him in.

"There was no way we were going to allow him to get away. We knew the case had to be watertight."

Days after Pacteau was charged, police divers recovered the spanner he used to batter Karen within the Forth and Clyde Canal near his West End home.

And despite the tool being underwater for almost 10 days, forensic scientists still managed to find tiny specks of Karen's DNA.

The scientific matches between the different sets of DNA samples and result from the soil samples showed Karen had been in Pacteau's car and he had killed her.

Experts said that any "coincidental match" was highly unlikely.