On the morning of July 15 1997, Gianni Versace was shot on the steps of his Miami mansion. The 50-year-old fashion designer was returning home with a selection of magazines bought from his local news café on Ocean Drive when he was twice hit in the head. Rushed to hospital with a faint pulse, his injuries proved too severe. At 9.20am, he was declared dead.

It sparked an international media sensation, a nationwide search for a killer – and one of the largest failed FBI manhunts of all time.

Two decades on, the shooting is the starting point for the latest outing of American Crime Story, the critically acclaimed television series that launched in 2016 with the ten-parter, The People vs OJ Simpson.

Like most people, that brief summary of Versace’s murder was more or less all that I knew when I was approached by Brad Simpson and Nina Jacobson, producers of American Crime Story, to write a mini-series about the events leading up to it.

They had responded to my novel Child 44, loosely based on the Russian serial killer Andrei Chikatilo, and my scripts for the BBC drama, London Spy. But the original idea for The Assassination of Gianni Versace had come from Ryan Murphy, the king of American television and creator of hit shows including Nip/Tuck and Glee.

I was sent a copy of Vulgar Favors, Vanity Fair journalist Maureen Orth’s book chronicling the months and years preceding the Versace murder. It was remarkable not least because it showed how little I knew about the complexity and heartbreak of the story. I was a crime writer, a reader of true crime, and this was one of America’s biggest murder investigations of all time – so why had it passed me by?

My first impulse as a scriptwriter when starting on a new project is to try and read everything written on the subject. In some cases, that is impossible; there’s simply too much. In this instance, it was with surprise and some dismay that I discovered how little material there was, both about the crime itself, but also about Versace as a man.

In terms of public profile, it was the very opposite to the OJ Simpson case. With that trial, most people knew its various twists and turns, the names of the lawyers, even actual lines of courtroom dialogue. With Versace, I didn’t even know there had been four other murders leading up to his. I didn’t know the names of these victims, nor their stories. What, if anything, connected them to Versace?

Far from being asked to dramatise a famous moment of history, the challenge felt closer to being asked to solve an untold mystery.

And so it was that, three years ago, I heard the name Andrew Cunanan for the first time, the young man with an IQ of 147, once full of promise and potential, who was ultimately responsible for five savage murders. ​Did he know Versace? It seems that they’d met in San Francisco four years before the murder. But what had happened between them?

When I asked Orth what had drawn her to the case in the first place, she answered that she’d seen a photograph of Cunanan, a handsome young man, wearing black tie, and it struck her that he seemed such an unlikely killer. This is the question at the centre of The Assassination of Gianni Versace: not who did it – there’s no doubt about that – but why he did.

Many killers display disturbing patterns of behaviour that go back many years. They’re violent, abusive, cruel to animals. Arson is a repeating indicator for a troubled psychology. If you had told Cunanan or his friends that, at the age of 18, he was going to be a notorious and despised killer, they would have found the idea impossible to believe.

Cunanan was a gentle boy with a high-pitch voice, mocked for being gay, an effete Oscar Wilde-like figure at his school, who used his wit to deflect the homophobic taunts he regularly received. His father, Modesto, had been born in the Philippines, joined the US Navy, earned US citizenship, came to America to live the immigrant dream of success, joining Merrill Lynch and using his handsome salary to send Andrew to one of the finest schools in the country: Bishop’s in La Jolla, San Diego.

Cunanan read widely, delighted in art and literature. He liked to laugh; even more, he liked to make other people laugh. He recited Robin William’s monologues to his friends and family. He wanted to impress people. He wanted to be happy. He wanted to be loved.

The series we set out to make was never going to be simply the life story of Versace, though we contrast his success with Cunanan’s failures.

But Versace’s was a vibrant success story, about the particular nature of an individual’s brilliance, not a crime story; those are about the nature of society – in this case, the destruction wrought on so many by homophobia. How do you survive in a society where many consider your existence to be a crime?

The Assassination of Gianni Versace was my first experience of dramatising real events. Yet there wasn’t an inordinate amount of detail to go on. There had been no murder trial, there were gaps in understanding the timeline of the killer, and the police investigations were never held up to much scrutiny.

We were trying to build a picture of events from a series of fragments, all that remained from the wreckage of lives destroyed by Cunanan.

So what was the connection between his five victims, who were killed during a three-month period in 1997: an aspiring young architect in Minneapolis, a former US Navy sailor, a Chicago real estate tycoon, a devoted national parks employee and a globally renowned fashion icon?

Cunanan had been on the FBI’s Most Wanted list for more than a month before the designer’s death, and was believed to be on the loose in the Miami Beach area. Why was the local community not warned? Why did it take his suicide, eight days after he shot Versace, to put an end to the killings?  

By dramatising the Versace story, my hope was that while I might make mistakes in the detail – for example, conflating characters for clarity, or giving characters lines of dialogue when we have no transcripts to guide us – such inventions would service the central themes and a larger truth. I raise this because The Assassination of Gianni Versace has come in for criticism from several quarters, in particular our decision to portray Versace (played by Venezuelan actor Édgar Ramírez) as having been HIV-positive.

Though his status was never made public in his lifetime, nor confirmed after his death, the suggestion that he was positive is prominent in Orth’s book, the primary resource for the show; to erase mention of it felt like removing part of the period’s history.

In many ways, the Aids crisis offers a parallel to Cunanan’s killings: gay men had been left to die while the world looked the other way, and it was only once a celebrity died that the world took action.

Part of what inspired me about Versace, in contrast to what appalled me about Cunanan (played by Glee star Darren Criss), was how one man overcame the obstacles in his life, while the other was consumed by hatred; how one man created while the other man destroyed. Andrew Cunanan was not a serial killer – he was a terrorist, a man filled with loathing for other people’s success. He saw himself as a victim of this world.

To that end, his journey is a road movie through American society.

20 years on, why are there so many unanswered questions about Gianni Versace’s murder?