Since Italian fashion designer Gianni Versace was shot dead outside his Miami Beach home in 1997, his murder has been pored over in countless articles, books and films. Now the shooting will be examined on television next year in the award-winning American Crime Story.

Amid the speculation, Antonio D’Amico, Versace’s boyfriend of 15 years and the person who found him sprawled on the steps of the mansion, has remained remarkably taciturn. But the forthcoming series, in which he will be played by singer and actor Ricky Martin, has spurred D’Amico to speak his mind.

“There has been so much written and said about the murder, and thousands of suppositions, but not a trace of reality,” D’Amico told the Observer.

The 58-year-old has not been consulted for the series, which will be entitled American Crime Story: The Assassination of Gianni Versace, and said the images he has seen online of how he reacted are incorrect.

“The picture of Ricky Martin holding the body in his arms is ridiculous,” he said, adding that it was like looking at a mimic of Michelangelo’s Pietà, which depicts the body of Jesus in the arms of his mother after the crucifixion. “Maybe it’s the director’s poetic licence, but that is not how I reacted.”

D’Amico says the tragedy of his lover’s murder on 15 July 1997 tipped him into a deep and long depression. Even now he will only briefly discuss it. Versace, who was 50 when he died, was killed shortly before 9am as he returned to his Ocean Drive home after buying a newspaper at a nearby cafe. D’Amico was drinking coffee on the veranda close to the mansion’s entrance when he heard the shots.

“I felt as if my blood had turned to ice,” he said. He and Versace’s butler went outside to investigate.

“The house had stained glass windows so we couldn’t see what had happened from inside, so we had to open the gate. I saw Gianni lying on the steps, with blood around him. At that point, everything went dark. I was pulled away, I didn’t see any more.”

Just days before, Versace had celebrated the successful launch of a collection at a show in Paris. He was shot by Andrew Cunanan, a 27-year-old gay man who had murdered at least four other people in a three-month killing spree before turning up at the fashion designer’s home. After a huge manhunt, the body of Cunanan was found eight days later in a Miami houseboat. He had apparently shot himself with the same gun that killed Versace.

Twenty years on, it is not known whether the murder was planned or carried out at random, leading to much gossip and speculation, including a rumour that Versace may have been murdered by the mafia due to debts he owed to the criminal organisation. There was also speculation that Versace may have met his killer years earlier.

Gianni and Donatella Versace in the 1990s. Photograph: Rose Hartman/Getty Images

D’Amico insists: “They never knew each other … so much has been fictionalised. Unfortunately Gianni died, unfortunately this guy killed him, unfortunately it happened: but now, let it drop.”

The murder tore D’Amico’s world apart. He went from partying with the likes of Elton John and Sting to shutting himself away in solitude. Meanwhile, tensions with the Versace family, in particular Gianni’s sister Donatella, put paid to him receiving what was left to him in the will – a monthly pension for life of about €26,000 and the right to live in Versace’s homes.

As the properties belonged to the Versace fashion house they came under the control of Donatella (to be played by Penélope Cruz) and older sibling Santo, as well as Gianni’s niece Allegra. D’Amico, who had been a designer at Versace Sport, received just a fraction of the pension and walked away from fighting for the rest. He then slipped into a depression lasting several years.

“I had never been through a depression and never saw a therapist as I was advised to: why did I need to tell someone else what had happened when I knew I was this way because Gianni’s death had torn me in two? I was in a nightmare, I felt nothing and gave no importance to anything … the house, the money … because it felt false to have expectations of life.”

In an interview last June, Martin, who came out as gay in 2010, spoke about a scene in the television show in which Gianni, played by Edgar Ramirez, is walking along the beach with Antonio when he suddenly becomes weak. His boyfriend touches him and Gianni responds: “Don’t touch me! The paparazzi!”

But Versace never tried to hide his sexuality, D’Amico said, and was one of the first people in the public eye to declare being gay in the late 1980s.

“We lived like a natural couple, there was never a problem,” he said. “It was the right moment for him to come out in public, but everyone involved in our world knew. He never tried to hide who he was.”

D’Amico doesn’t plan to watch American Crime Story but said he’d be happy if Martin got in contact to get some insight into his relationship with Versace. “It’s getting to know the small things about a relationship … for example, Gianni was so ordered and focused at work but in his private life everything was disorganised. He’d leave the bathroom in a mess. At a certain point I said ‘enough’! And when it came to cooking, he didn’t even know how to [boil] an egg.”

D’Amico eventually emerged from his depression after deciding it was a question of either starting to live again or stopping completely. He found love again in 2005, and now lives a simple life with his partner in the northern Italian countryside. He has also relaunched himself as a designer, recently bringing out a collection of sportswear for the golf sector.

“Sincerely, after two decades, I will always be connected to Gianni as a person I loved for more than 15 years,” D’Amico said.

“But today, I am a different person … the world continues to go around … You can look back at the past until a certain point, [but] then you need to look ahead to the future.”