Such is the level of suspicion in this story that even the date of death is deemed proof of a conspiracy. Twenty years ago, JonBenét Ramsey, a six-year-old girl known for ever to the world by the uncomfortably adult poses she struck in her beauty pageant photos, was found bludgeoned and garrotted in her family’s basement in Boulder, Colorado. The killer has never been found and, ever since, the case has been picked over by experts, the tabloids and an endless slew of internet obsessives.

It is impossible to overstate how huge this case was – and still is – in the United States. Every year, the US media promise “A chilling new discovery” and “Latest twist”, even though the case remains as cold as Christmas in Colorado. There is now not one single part of this sad tale that has not been seized on – by the public, by the police, by JonBenét’s parents, John and Patsy – as proof of a coverup, even down to the child’s gravestone near Atlanta, Georgia, where she was born. There, the date of death is literally carved in stone: 25 December 1996. Yet even this is seen as a lot less stable than its material suggests. After all, doubters say, how could John and Patsy have known that their daughter died on Christmas Day if they didn’t find her body until the early afternoon of the 26th? Surely the gravestone is evidence of their guilt that so many have long assumed, despite them being exonerated in 2008 by DNA evidence?

The few undisputed facts are as follows: just before 6am on 26 December, Patsy called the Boulder police department from her home. Her daughter had been taken from their home in the middle of the night, she said. She had found a two-and-a-half-page ransom note demanding $118,000 for her safe return. The police arrived at the Ramsey house, along with many of the Ramseys’ friends, who wandered freely around the property. After the kidnappers failed to call at the promised time, one of the officers suggested to John that he look around the family’s large house. He went down to the basement with a friend and there he saw his daughter, bound and gagged. When he brought her upstairs, it was obvious she had been dead for some time. She had been bashed over the head, strangled with a garrote fashioned out of a nylon cord and her mother’s paintbrush, and possibly sexually assaulted. There was no immediately obvious sign of a break-in, and the house was so large, the perpetrator must have known its layout very well to have found JonBenét’s bedroom in the middle of the night and taken her down to the basement without waking anyone else.

Some historical crime stories fascinate the public years later because of what they say about the times in which they happened. The OJ Simpson case and the Manson killings are two obvious examples, both of which have experienced a revival of interest this year, thanks to their retelling in pop culture. The story of JonBenét Ramsey is different.

The case is certainly in the spotlight again, with three US networks – CBS, A&E and Investigation Discovery – all recently screening their takes on the case to varying degrees of tackiness. The media coverage of this case was, and remains, almost unparalleled in its tawdriness. Photos of the little girl’s autopsy were bought and published by US tabloid the Globe. One journalist claimed to convert from Judaism to Christianity in order to attend the Ramsey’s church and glean insights from staring at the back of John and Patsy’s heads. (“I had never seen anyone pray for his own soul the way Patsy was praying for hers … At that moment, I decided she was the killer,” the journalist, Jeff Shapiro, said in probably the best-known book about the case, Lawrence Schiller’s Perfect Murder, Perfect Town.) The whole story has long been covered in a thick sheen of schlock. The only interesting thing about its historical context is that the case happened in the aftermath of the OJ Simpson trial, just when the media was desperately seeking another case that would similarly hold the public’s attention.

But while JonBenét’s murder may well never be solved, there has never been any mystery about why it still exerts such a fascination. Like Madeleine McCann, JonBenét was a pretty blond child from a well-off family, allowing the public the pleasure of looking at this photogenic child while simultaneously experiencing a quiet, unacknowledged frisson of schadenfreude at her parents’ pain. Add to this the undeniably sexualised pageant photos, in which the six-year-old wiggled and pouted in a manner that looked all the more fascinatingly horrific after her brutal death, and you have the perfect made-for-media confection. But none of these factors really explains the passion this case excites.

Pretty much from the moment this story broke, there have been two theories about what happened: either JonBenét was killed accidentally by one or both of her parents, or her then nine-year-old brother, Burke, and the parents staged a faked kidnapping to cover up the killing; or it was a botched kidnapping by a mystery outsider. Part of the reason this case has never been solved is because the Boulder police department badly bungled the first few days. Understaffed during Christmas and wholly unprepared for such an extraordinary case, they failed to secure the crime scene. They didn’t even find JonBenét’s body in the basement, leaving John to do so hours later. Their relationship with the Ramseys completely broke down when they threatened to refuse to release JonBenét’s body for burial unless the Ramseys came in for an interview, and there were frequent leaks from the Boulder police department to the media. “The police were not there to help us. They were there to hang us,” John Ramsey has said since. A 2015 Reddit ask me anything (AMA) discussion with former Boulder police chief Mark Beckner did little to disprove the Ramseys’ belief that the police continue to assume they were guilty. (Beckner has since expressed regret for the AMA, saying he hadn’t realised it would be made public.)

JonBenét Ramsey. This and other photographs of her competing in beauty pageants helped to turn the public against the family after her death. Photograph: Sipa Press/Rex Features

“The biggest mistake in this case is that there was a phenomenal number of people who decided on the first day that they knew what happened, and they would not allow new information to change that, and that boggles my mind,” says journalist and Boulderite, Charlie Brennan, who has been covering the story from its first days.

However, it is understandable why the police suspected the Ramseys. When a child is killed at home, it is statistically likely that a parental figure was involved. The Ramseys, according to the police, were reluctant to be interviewed (the Ramseys have vociferously denied this), and John was overheard on the phone an hour after finding JonBenét making arrangements for his family to leave the state (he has since said he was just trying to keep them safe). They were also swift to hire lawyers – suspiciously quick in the eyes of many. As for the date on her gravestone, her parents have since said they chose it because that was the last time they saw their daughter.

“The Ramsey case is a criminal Rorschach test – every piece of evidence can be looked at in several ways, and I’ve never seen that in any other case,” says Brennan. One example of this is the weirdly long ransom letter, demanding a very specific amount of money, which happened to be almost exactly what John Ramsey had received as a bonus that year. Even weirder, the note was clearly written in the Ramseys’ house, using a pad of paper and pen that were there. To some, this proves the Ramseys must have written it: what kidnapper would hang around to write such a long note? But to others, this is proof that they didn’t. Why would the Ramseys mention John’s bonus in the letter? It must have been someone with knowledge of his well-publicised business affairs who wanted to hurt him. The killer could have broken in while the family was out on Christmas Day visiting friends, their defenders say, and written the note while they were out, lying in wait until they went to sleep.

Then there are the pageants. Without question, the images of the little girl sashaying on stage in makeup, which were released without her parents’ consent, helped to turn public opinion against the Ramsey family. It certainly turned many people in Boulder against them.

“Boulder sees itself as a very sophisticated community and a lot of people figured that this whole spectacle was beneath them,” says Brennan. “The Ramseys had only moved to Boulder a few years earlier [from Georgia], and then the whole pageant thing came to light, which was something that was completely foreign to most people in Colorado and more associated with the deep south. So a lot of Boulderites felt: ‘This does not reflect us, they are not one of us.’” As a result, Brennan says: “A lot of people in Boulder feel there is no mystery – they know who did it.”

Even JonBenét’s parents seemed divided about the pageant issue. In their book about the case, The Death of Innocence, that they wrote together in alternate sections, John insists beauty pageants were just “one of her many hobbies”. Patsy, however, spends the next seven pages describing her daughter’s “gift” at “performing” (she also doesn’t mention any of her daughter’s other hobbies). Beauty pageants were unusual in Colorado, she writes, but she had done them herself when she was younger. For one, she bought JonBenét “a Ziegfeld Follies costume, reminiscent of the one I had worn in the Miss West Virginia Pageant some 20 years earlier. Like mother, like daughter.”

Protesters unhappy at the lack of a decision in the murder case march outside the justice centre in Boulder, Colorado, in 1999. Photograph: Reuters

So Patsy, in particular, was shocked by the negative public reaction to the pageant photos after her daughter’s murder. One newscaster said the six-year-old resembled “a hooker”. Pretty much only pageant photos of JonBenét were used in the media, even though she had only been in nine pageants. There was a definite intimation in the now-hysterical media coverage that to put your child in a beauty pageant was weird, unnatural and sexually suspect. JonBenét was simultaneously deified as a photogenic angel and vilified as a child temptress, and her parents were criticised for fetishising her looks, while the public and media did exactly the same thing themselves. “What I saw on the pageant video … you don’t do that to a six-year-old,” JonBenét’s former dance teacher, Kit Andrew, says in Perfect Murder, Perfect Town.

But there is an alternative way of looking at the pageants. Child beauty contestants, while unusual in Colorado, are hardly unknown in the US. Thousands of pageants still take place every year, and no one is saying every parent involved is a potential killer. In fact, far from incriminating the Ramseys, the pageant photos could be seen as almost exonerating them: it could very easily be argued that the pageants brought JonBenét to the attention of a local paedophile, and several have since been suspected, but never charged.

John and Patsy’s general demeanour was also deemed suspicious by the police, the media and the public. “The Ramseys didn’t appear to behave the way parents in this situation are ‘supposed’ to behave. They didn’t cling together and constantly comfort and reassure each other,” John Douglas writes in The Cases That Haunt Us.

But John and Patsy were, they write themselves, “in shock and medicated so we could function” for weeks after the murder. So to judge how they spoke, looked and interacted as being indicative of something was not really fair. But this is what happens to every parent who loses a child in a high-profile case: their behaviour is scrutinised for clues.

When a parent loses a child, the most natural human response is sympathy. But that is not what many feel for parents in high-profile cases. When Madeleine McCann went missing in 2007, her father, Gerry, and in particular her mother, Kate, were widely criticised: Gerry, some sniped, was too articulate and Kate looked too pretty. What kind of mother puts on eye shadow when her daughter is missing? Kerry Needham was dismissed as a feckless teenage mother when her baby son, Ben, went missing on Kos in 1991. When two-year-old Lane Graves was killed by an alligator at Disney World in a freak accident earlier this year, parenting chat sites were swamped with people criticising the parents for letting a little boy play near the water in the evening, as though that were unusual on a Florida holiday.

Parent-blaming is all-too-common these days, and usually the point is to make other parents feel better about their own parenting skills. But in cases such as that of JonBenét, something else is going on. By demonising parents who have suffered a terrible trauma, the rest of us can reassure ourselves that they are different from us: those parents are flawed, even evil, and we are good and therefore our child will never go missing – in Kos, in Praia de Luz, from our house in the middle of the night – like theirs did. The rush to blame JonBenét’s parents can also be partly put down to the public needing to reassure themselves that, contrary to what the Ramseys said, killers don’t break into houses and murder children where they should be most safe. That only happens when the parents themselves are killers. And yet.

Brennan says: “In 2000, I wrote a piece that ran in the Dallas Morning News pointing out that, nine months after this crime, someone broke into a house near the Ramsey house and was in the process of assaulting a nine-year-old girl in the middle of the night and was chased out by her mother. The girl went to the same dance studio as JonBenét. The police said they believed it had no connection to the Ramsey case.”

After writing about the case for 20 years, Brennan says he has come to believe the family weren’t involved: “If you look at the autopsy photos and you see the deep furrow in her neck created by that ligature, you see a tremendous amount of force was used. That does not suggest staging to me – the person who did it, meant it. But the Ramseys have nothing in their background to suggest that this level of evil dwelled in their hearts,” he says. But this theory, like the ones about whether the Ramseys behaved how they were “supposed” to, relies on imagining how we would behave if our child had been killed, or if we had killed them accidentally. But no one can do that accurately. And anyway, it’s irrelevant, since the case is about the Ramseys, not anyone else.

It is entirely possible JonBenét was killed by a member of her family. It is also very likely the case will never be solved: Patsy has since died and the case gets colder every year. The ghoulish hysteria around her murder has lasted more than three times longer than JonBenét’s life did. “I’ve covered lots of big stories: the Challenger, presidential elections. But this – it is something that I’m thinking about all the time,” says Brennan. “It is an impossibly complex, seemingly unsolvable riddle.” It is also the death of a child, killed with shocking brutality. But it’s hard to see the truth beneath the schlock.

The Killing of JonBenét: Her Father Speaks, 9pm, 11 December, Crime and Investigation; Who Killed JonBenét?, 10pm, 18 December, Lifetime; The Case of JonBenét Ramsey, 22 December, More4