WHEN a plumber was called to unblock a clogged drain on a February afternoon in 1983, little did he realise that he was about to stumble upon a dark secret – one that would unmask Scottish serial killer Dennis Nilsen and a raft of heinous murders.

Fraserburgh-born Nilsen was living at 23D Cranley Gardens in leafy Muswell Hill, North London, when the discovery of human flesh and bones were made on the property.

Later, when Nilsen was being escorted to the police station, the seemingly mild-mannered civil servant was asked whether the remains belonged to one person or two. Staring out of the window, he replied, "15 or 16, since 1978".

It was a killing spree that sent shockwaves across the country, not least when Nilsen's calm and methodical method of disposing of his victims was revealed, along with disturbing details of his necrophiliac tendencies.

The chilling story has been made into an ITV drama, Des, starring David Tennant as Nilsen. Daniel Mays plays arresting officer Detective Chief Inspector Peter Jay, with Jason Watkins as Brian Masters, who corresponded with Nilsen in prison and wrote the book, Killing for Company, which inspired the three-part series.

Here, we delve into the murky life of Nilsen – who used the name "Des" as he lured his victims – and examine what makes a serial killer tick.

His early years

Dennis Andrew Nilsen was born on November 23, 1945 in Fraserburgh, Aberdeenshire. He was the second eldest of three children. His parents were Elizabeth Duthie Whyte and Olav Magnus Moksheim (who had taken the surname Nilsen), a Norwegian soldier who came to Scotland in 1940.

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It was a troubled marriage and Olav spent little time with his family. The couple divorced in 1948. Elizabeth and the children remained living with her parents. Nilsen's early childhood memories were of family picnics and long walks in the countryside or along the harbour towards nearby Inverallochy.

READ MORE: Professor David Wilson: 'Dennis Nilsen was different to the other serial killers I have met ...'

His maternal grandparents led a pious life (referred to by their grandson as "cold and dour"), yet Nilsen forged a close-knit bond with his grandfather, a fisherman who he viewed as a "great hero and protector". When his grandfather was off at sea, Nilsen would pine until he returned.

A few weeks before Nilsen's sixth birthday, his grandfather died of a heart attack while fishing in the North Sea. Nilsen would later describe seeing the open coffin at the family home as his most vivid childhood recollection.

A troubled teenager

Following the death of his grandfather, Nilsen is said to have become quiet and withdrawn. When puberty arrived, Nilsen realised he was gay. However, feelings of confusion and shame led him to hide his sexuality from family and friends.

At 14, Nilsen joined the Army Cadet Force, eyeing a career in the British Army as a possible escape from the constraints of his rural upbringing.

Life without an anchor

Nilsen enlisted in the Army in 1961 and trained as a cook. His 11-year military career saw postings to Germany, Norway, South Yemen, Cyprus and Scotland (Inverness and Shetland). 

After ending his military career in 1972, Nilsen moved to London and briefly joined the police, before finding work as a security guard and then a civil servant in a Jobcentre.

The murders

Dennis Nilsen murdered at least 12 young men and boys in London between 1978 and 1983. He attempted to kill seven others and initially confessed to there being 15 victims.

Nilsen targeted homeless people and homosexual men, typically strangers he met in bars and on public transport who were enticed to his home with offers of alcohol or a place to stay.

All the killings took place at two North London addresses – before Cranley Gardens, Nilsen lived at 195 Melrose Avenue.

His modus operandi

After strangling his victims until they were either dead or unconscious (those who passed out were drowned in a bathtub), Nilsen would observe a ritual in which he bathed and dressed the body.

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He retained the corpses for several weeks, sometimes months, before dissecting and disposing of the remains on a bonfire or flushing them down the toilet. He also boiled the heads, hands and feet of his victims to remove the flesh.

A gruesome discovery

Curiously, Nilsen played a hand in his own downfall after he wrote a letter of complaint to his landlord in February 1983 saying that the drains at Cranley Gardens were blocked. An emergency plumber made the grisly find of a flesh-like substance and some small bones.

He reported the discovery to a supervisor, who arrived that evening. Nilsen and a neighbour joined the men as they voiced fears it might be human remains. To which Nilsen replied: "It looks to me like someone has been flushing down their Kentucky Fried Chicken."

By the next morning, the drain had been suspiciously cleared but evidence of further flesh and bones were found in a pipe. The police were called, and a pathologist confirmed the remains were human. One piece of skin was from a neck bearing a ligature mark.

Nilsen initially feigned shock but when told by DCI Peter Jay, "Don't mess me about, where's the rest of the body?", he responded calmly, admitting that it could be found in two plastic bags in a wardrobe.

One of these contained two dissected torsos, alongside a smaller shopping bag with assorted internal organs. The second held a human skull almost completely devoid of flesh, a severed head, and a torso with the arms attached but its hands missing.

The confession

In a police interview, Nilsen said there were further remains inside a tea chest in his living room and an upturned drawer in his bathroom. The dismembered body parts belonged to three men, all of whom he had strangled to death.

READ MORE: Professor David Wilson: 'Dennis Nilsen was different to the other serial killers I have met ...'

Charred bones from at least eight bodies were found at his previous address in Melrose Avenue. During police interviews, Nilsen remained adamant about not understanding his own motives for the murders, saying: "I'm hoping you will tell me that".

Sentencing and life in prison

Nilsen admitted killing at least 15 people but ultimately was convicted of the murders of six men and two attempted murders. He was sentenced to life imprisonment on November 4, 1983. Nilsen died in York Hospital on May 12, 2018, aged 72, after collapsing in his prison cell.

Des begins on STV, Monday, at 9pm