In 1983, Brian Masters reached out to one of Britain's most notorious serial killers.
The author, who was best known for his books on British aristocracy, had never previously had any interest in the inner workings of mass murderers.
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Nilsen, who is currently the subject of Stan's new miniseries Des, was known throughout Britain as the Muswell Hill Murderer.
Between 1978 and 1983, Nilsen killed at least 12 young men and boys in London, and attempted to kill seven others.
Typically, Nilsen met his victims – who were largely homeless and/or homosexual men – in bars or on public transport, before inviting them back to his home with the promise of alcohol and shelter.
Once at his home, his victims were typically strangled and then drowned, before being stowed under the floorboards and later disposed of.
Eventually, Nilsen was apprehended by police. When asked by police if there were any body parts in his home, he responded: "It's a long story; it goes back a long time. I'll tell you everything. I want to get it off my chest. Not here — at the police station."
He also confessed to murdering "15 or 16" young men, while showing no remorse.
As the case hit the media, Brian Masters wrote to Nilsen.
The author was interested in how "an articulate and hard-working employment agency officer" could become a serial killer.
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Masters was also interested in the public's reaction to the case, as many headlines linked the murders to London's gay scene, as Masters is gay himself.
"My first letter from him began with a disconcerting sentence: 'Dear Mr Masters, I pass the burden of my life onto your shoulders,'" Masters wrote in his book, Killing for Company, according to GQ.
As seen in Des, Nilsen and Masters later met for the first time in prison.
"My first visit was dominated by Nilsen’s body language. We sat at a square table, with a tin ashtray in the centre which he filled throughout the interview — cigarettes being the only luxury I was permitted to bring with me," Masters wrote exclusively for The Sun in September.
"His arm hung casually over the back of his chair, as if he were in charge and was inviting me to explain myself. That attitude was to change as he gradually realised that I was not so easily manipulated and intended to manage the relationship in my own way."