COLD and calculating killer David Gilroy drove his family to dinner in the car where Suzanne Pilley's body had been dumped just hours earlier.

Prosecutors convinced the jury in Gilroy's murder trial at the High Court in Edinburgh that he killed Ms Pilley during a violent struggle in the basement of their workplace in the capital and then hid her body in the garage.

He is then believed to have calmly gone home to get his car, evaded other staff as he dumped her body in the boot and continued about his business as if nothing had happened.

The killer is thought to have ferried his family about Edinburgh that night in his Vauxhall Vectra, attending a presentation at his child's school before going for a meal at Vittoria's restaurant in Leith Walk.

The following day he drove to Lochgilphead on a trip he didn't need to make, and is thought to have disposed of Ms Pilley's remains somewhere between Inverary and Tyndrum – a stretch of the journey that should have taken about 40 minutes but took Gilroy two hours, and an area where the couple had spent romantic breaks together.

But lack of a body or crucial DNA meant officers had to pull strands of circumstantial evidence together to form a compelling case against Gilroy.

About 70 officers worked on the investigation and police said the case will remain open until the body is discovered.

It is also possible Ms Pilley's body was disposed of under- water, with miles of Loch Fyne shoreline and other lochs within reach, although initial investigations led police to land searches.

Lothian and Borders Police were helped by members of the Arrochar, Lomond, Oban and Tweed Valley mountain rescue teams, and Strathclyde Police gave logistical support.

Hundreds of miles of rugged terrain was painstakingly searched. More than 200 people were involved in scouring some 100,000 acres of land.

The hunt focused on the Argyll Forest, targeting Glen Croe, and after one of the public appeals, an area known as Hell's Glen – a valley off the B839 that is accessible by a forestry track – was also scoured.

Det Supt Gary Flannigan, who headed the inquiry, said: "Investigating the murder of Suzanne Pilley was groundbreaking in many respects for Lothian and Borders Police. It started as a missing persons inquiry and quickly transformed into a murder inquiry."

It was the first time Lothian and Borders Police used a "cadaver dog" trained to locate human remains, and it pointed to there having been a body in the basement and his car.

The extensive use of electronic data was also key to the investigation and officers used CCTV and mobile phone triangulation to help build their case. The search focused on a "huge area" in Argyll and was based on sightings of Gilroy's car by members of the public.

But the key came in Gilroy's own behaviour.

Despite showing his "cold personality" in the immediate aftermath of the killing, he made a number of mistakes.

In the weeks running up to the murder he had sent her 400 texts: on one day there were 64 texts and four calls, and on another there were 57 texts and seven calls. While friends and family continually rang her mobile phone after her uncharacteristic disappearance, Gilroy made no such attempt. This was because he had the phone and knew she was dead, police said.

The couple first got together in 2009 after he had left his wife and he moved into her flat in Stenhouse a few months later.

The on-off relationship was fraught with difficulties and Gilroy displayed his volatile temperament when he threatened a neighbour, who called police after the killer "went berserk" and threw objects out of the flat window.

Officers who examined hundreds of hours of CCTV footage from around the Thistle Street area where they both worked were able to establish that Ms Pilley never left the area within normal view.

She was captured on CCTV getting off a bus at Jenners and going to a nearby supermarket on the morning of May 4, 2010. She was making a "routine journey to work" but was never seen again.

Crucial pointers were to follow. When police visited her home they found her passport, some cash and medication. There was nothing missing and nothing disturbed. Her family said there was nothing out of the ordinary in the house, and it looked like she could have walked through the door at any moment.

Gilroy bought air fresheners after the killing, and a colleague commented on the overtly fresh smell in the vehicle.

None of her bank accounts were touched and her credit cards have never been used.

Prosecutors said these facts, brought together, demonstrated Suzanne was dead.

Gilroy was first interviewed by police as a witness when it emerged the two had a relationship.

Police phoned him on his mobile the day after she was reported missing and he told them he was in Lochgilphead, Argyll, on business. He returned to Edinburgh late that night and gave a statement to officers.

His statement ran to 59 pages after an 11-hour interview.

A bizarre attempt to cover up scratches on his hands and body with make-up raised the suspicions of officers and Gilroy was asked to attend a forensic medical examination the next day.

They believed he was covering the cuts with a "flesh-coloured substance", possibly make-up.

This was the "turning point" in his status, police said.