It's the unsettling mix of the incredibly intimate and the coolly tactical that has made the mystery of Madeleine McCann the biggest and most extraordinary child abduction story in history

Madeleine McCann: a vibrant three-year-old frozen in time. Photograph: David Moir/Reuters

In November, 30 months after their daughter vanished from their holiday apartment, Kate and Gerry McCann released two images of how Madeleine might look now. Her face is a little longer, her jaw stronger; she has grown out of her toddler's button nose. In one, she is shown with long blond hair, in the other with darker hair and a deep suntan. But in both images she retains the distinctive black flash in her right eye where the pupil bleeds into the iris, and which they hope can be used to identify her, if they ever do succeed in finding her.

Madeleine's distinctive eye has been central to the search for her since the earliest days. The couple released posters in English and Portuguese in which the letter "o" had been modified to have the same distinctive flash. "Look into my eyes," read the images: "Olha para os meus olhos." There were rumours that the Bryan Adams song "(Everything I Do) I Do it For You", which begins with those words, would be re-released in support of the campaign.

Late in 2007, Gerry McCann gave an interview to an American magazine and talked about the decision to publicise the eye defect. "Certainly we thought it was possible that [the publicity] could possibly hurt her or her abductor might do something to her eye . . . But in terms of marketing, it was a good ploy."

It is this unsettling mix – of the incredibly intimate and the coolly tactical – that has made the mystery of Madeleine McCann arguably the biggest and most extraordinary child abduction story in history. HL Mencken, the great American essayist and reporter, called the 1932 disappearance of the baby son of aviator Charles Lindbergh "the biggest story since the Resurrection", but neither the Lindbergh baby kidnap and murder, nor Christ's rising from the dead, took place in the internet age.

Just a few weeks after she vanished in May 2007, a sizeable chunk of the globe knew the name Madeleine McCann. The rather homespun website set up by her parents had 80m visits in the first three months after her disappearance. Millions of pounds were offered in reward for information. The biggest celebrities in the world – David Beckham, JK Rowling, the Pope, Oprah Winfrey – publicly expressed support or interest in this anonymous middle-class couple from the Midlands.

Reporters and camera crews from around the world descended on the small Algarve town of Praia da Luz, to feed an audience desperate for updates. At one point, almost two-thirds of global traffic on Google News consisted of searches for information about Madeleine. Most remarkable of all is that despite the many thousands of articles, the millions of words, written about Madeleine McCann, there remains more than two and a half years later just one solitary fact that we know for sure. In the early hours of 3 May 2007, she vanished without trace from her parents' holiday apartment.

Madeleine was not the first helpless child to come to harm, nor, tragically, will she be the last. So why did this child, this story, become the one that convulsed the world rather than any other? In part, it may be because the McCann case speaks to a profound noughties unease about the rules and roles of parenthood. Would you leave your three children asleep in a strange apartment while you dined and drank with friends in a restaurant some distance away? Have you? Would you heed the advice not to weep in public if your child was taken? How composed is too composed?

Kate and Gerry McCann, so profoundly conventional in many ways, awkwardly resisted conforming to the behaviour that an increasingly engaged and judgmental public demanded, most notably a stubborn refusal to acknowledge any parental culpability on their part and a determinedly dry-eyed public face, albeit on the advice of professionals, that sat uneasily with the sentimental grammar of tabloid reporting and the public mood.

In the case of Sarah Payne, snatched and killed in July 2000, or of Milly Dowler, who vanished in March 2002, or of Holly Wells and Jessica Chapman, who died five months later, the threat was external and unforeseen. Baby P, who died three months after Madeleine vanished, was murdered in circumstances of unambiguous evil. Terribly unjust as it may be, Madeleine's parents' dreadful victimhood was complicated, in the mind of the public, by their parenting decisions. It set in play the circumstances that allowed their critics, for a time at least, to judge them more harshly than whoever snatched her.

But the story of Madeleine is also a story about the media, how news events are set in motion, and how the plates are kept spinning, and how sometimes they fly off uncontrollably in all directions. A beautiful toddler gone missing will always be catnip to newspaper editors, but Kate and Gerry McCann also chose to make themselves active characters in the story, and though their motives were laudable, their relentless drive for publicity unsettled many. Had Madeleine been snatched in Britain, the McCanns would have been assigned a police family liaison officer and the full, slammed-door stonewalling of a police press office. In Portugal, their advisers were PRs. In October 2007 Clarence Mitchell, by then working as the couple's full-time media adviser, addressed students at Coventry University about the case. The title of his talk? "Missing Madeleine McCann: The perfect PR campaign".

There is another reason, of course, why Madeleine has become so iconic, and that is the terrible, ongoing mystery. "Madeleine is a very happy little girl with an outgoing personality," reads a heartbreaking note on her parents' website. "Like most girls her age, she likes dolls and dresses (and anything pink and sparkly)." What on earth became of this vibrant three-year-old, frozen in time? Will we ever know? Is it possible, as with Jaycee Lee Dugard or Natascha Kampusch or Elizabeth Fritzl, that one day a woman who was once named Madeleine will emerge, blinking, into the media spotlight?