In December 1999, a grim discovery is made during a routine police diving exercise in the frigid waters of Loch Lomond.

Divers notice the submerged garbage bags, expecting them to be filled with garbage – but on closer inspection, they find human limbs instead. Eight days later, another bag – containing a head – is found washed up 60 miles away on Barassie Beach, Ayrshire.

This six-part documentary series tells the inside story of a murder investigation that shocked the country. Senior police officers, including detectives at the heart of the investigation, experts and crime journalists give their insights into a horrific crime and the pursuit of justice.

In this opening episode, the focus is on the gruesome discoveries in the lake and on the beach, and the disappearance of a young worker. The unexpected discovery of human remains in a stretch of water that is twenty-two miles long and four miles wide is described as a “one in a million chance.”

The victim has been named as Kilmarnock youth Barry Wallace, who was last seen on a work night in his home town. A huge police operation is launched as a media frenzy ensues, with newspapers and broadcasters highlighting all the twists and turns of the grisly story.

Detectives soon identify a ‘person of interest’ – William Ian Beggs, born in Northern Ireland, who was known to Northumbria Police and had moved to the Ayrshire town.

Limbs in the Loch: Catching a Killer is a Firecracker production for BBC Scotland. All episodes are available on iPlayer from Tuesday 5 November.

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