Veronica crept up on us quietly. The Spanish horror movie landed on Netflix with little publicity at the beginning of last week, but has since become a word-of-mouth sensation. Viewers took to social media to complain that it had left them quaking and sleepless, prompting headlines asking whether it was the “scariest movie ever”.

Why has it proved so chilling? Because it's based on real events. And unlike so many horror films that claim to be inspired by a true story, Veronica has official documentation to back it up, an account of an incident so strange even hard-bitten police officers were forced to blame it on the supernatural.

The titular protagonist, played by Sandra Escacena, is a young girl who experiments with an Ouija board during a solar eclipse. Asper usualfor horror movie characters who dabble in the occult, her attempt to speed-dial the spirit world unleashes horrible forces into her life.

The film – directed and co-written by Paco Plaza, best known for his 2007 zombie hit REC – tracks poor Veronica’s fate across the course of the next three days. To tell her story, Plaza drew on the real-life case of Estefania Gutierrez Lazaro, a young girl from the Vallecas neighbourhood of Madrid who tragically and mysteriously died in August 1991.

Like her on-screen counterpart, Estefania, who was 18, supposedly used a home-made Ouija board early in 1991, attempting to summon spirits with while at school (some versions claim she and her friends were trying to contact a boyfriend who had died in a motorcycle accident).

The story, which has been recounted by Bustle, Newsweek and The Sun since the film’s release, alleges that after the group’s occult activities were interrupted by a teacher, a strange smoke was seen escaping the Ouija board glass and entering Estefania’s mouth.

Across the next six months (in contrast to the film’s three days) she began to behave increasingly strangely, telling her parents that she was being visited by faceless, emaciated figures who would call her by name. Her mother had a idea of who to blame. Estefania’s grandfather had died shortly before the strange events began. His delirious, dying wish – so the story goes – was to make his family's lives a living nightmare. 

Some retellings of the story claim that Estefania behaved erratically and later suffered convulsions, and most agree that she eventually fell into a coma before dying, on August 14, in the presence of her family. Reports differ, however, as to whether her death took place at the Gregorio Marañón hospital in Madrid, or her own bedroom. An autopsy, reportedly carried out by a Dr Pedro Cabeza, concluded that she had died of cardiac arrest – but the exact cause of her previous illness remained unknown. One report quotes her doctors describing her death as "sudden and suspicious".

After Estefania's death her parents, Concepcion and Maximo Lazaro, allegedly began to experience strange phenomena in their home, including disturbing voices, screaming sounds and an incident in which a framed photograph of Veronica spontaneously caught fire.

They eventually called the police, in the early hours of the morning on November 27 1992, after the hauntings took a violent turn – and it was this call-out which would eventually make the case famous in Spain. The visiting officers witnessed several strange incidents themselves, and were forced to record them in their write-up, providing an official record of what they believed to be paranormal activity.

"In Spain it's very popular, this story, because it is – as we say in the film – the only time a police officer has said he has witnessed something paranormal, and it's written in a report with an official police stamp and it's really impressive when you look at it,” Plaza said during a Q&A last year at the Toronto Film Festival.

So what exactly did the officers record? According to the report itself, featured on several Spanish-language sites, they witnessed a closet or wardrobe door opening of its own accord, some unsavoury-sounding “brown drool”, an incident in which a wooden crucifix had spun round, “tearing out the Christ attached to it”, and the mysterious materialisation of a three-clawed scratch on a poster on a door. 

Over time, some of the precise facts about the case have been muddied. Details – such as the place of Veronica’s death, or the exact nature of her experiences – vary from one telling to another. But this police report seems to be the one verifiable constant, and it wasn't just the testimony of an easily frightened young cop. José Pedro Negri, the officer who responded to the call-out, was the chief inspector of the national police.

Perhaps unsurprisingly, the family later decided to move out of the house. The new tenants, according to the Huffington Post, never experienced any strange phenomena.

Many recent retellings of the story seem to draw their facts from a 2012 episode of the Spanish television programme Cuarto Milenio (Fourth Millenium), a paranormal investigations show presented by Iker Jiménez, a Spanish journalist who specialises in exploring “the unexplained”.

Dig a little deeper, however, and you'll find a 1992 report into the hauntings from Spanish TV news station Antena 3. At first, it makes for unsettling viewing, featuring footage of Estefania’s parents as they show investigators and psychic experts round their home. In the video, Estefania's mother is adamant that her daughter was "possessed".

The news report includes a recording of electronic voice phenomena (EVP) supposedly heard at the house, saying the words "cuidado con el abuelo" – "watch out for grandpa".

The footage climaxes with a scene in which one of the ghost hunters is attacked by a "possessed" medium, who repeatedly asks him "do you know me?" In hindsight, it all looks clearly staged – and it's surprising that viewers took it seriously at the time. A reporter for Spanish news site El Español, studying the footage last year, noted that these ghost-hunting shenahnigans would be "unimaginable" on any mainstream TV news channel today. 

In a few years time, we might look back at Victoria the same way. It's certainly a well-crafted, intensely focused film, with a solid sense of unease and dread building up throughout, and some good heart-in-mouth, hand-in-front-of-eyes sequences. But the “scariest movie ever” label isn't entirely justified – and nor is the wholly true to call it a "true story".

While Paco’s film was loosely inspired by the real-life tragedy and subsequent reports, the director never intended it to be a definitive, entirely factual account. “You only have to read the different newspapers to know how different reality is, depending on who's telling it,” he said in Toronto. “So I knew we were going to betray the real events.”

The true story behind Veronica is sad, strange and (depending on your levels of scepticism) fairly disturbing – but the film's scares are all its own.