The strange ideas and enduring mysteries of the Kercher case

Amanda Knox is free, but we are still no nearer to learning the full truth

Amanda Knox has flashed through our lives in the past week, mutating from a pretty, stressed-out jailbird to the crumpled, crying heap we saw in court at the verdict last Monday night, to a beaming face at an airport and then to blessed invisibility.

The 24-year-old co-ed from a broken but comfortable home in Seattle really got under our skin, and she's still there. Yet the vastly different interpretations of her character and actions before, during and after the brutal murder of her flatmate Meredith Kercher, bring home how hard it is really to know anybody.

To one person, a smooth, nicely proportioned, girlish face and a ready smile mean innocence and transparency; to another, those same qualities may connote cunning, calculation and depravity. How we read them depends on our personal prejudices. Prosecutor Giuliano Mignini insists he is still convinced that the pair are guilty. In contrast, Mauro Chialli, one of the jurors, said last week, "I saw the faces of these kids, and they couldn't bluff. They didn't bluff." The accusation against them didn't convince him, he said, because "it was based on so many conjectures".

What the verdict means

The appeal court's judgment read simply: "They did not commit the crime." But in interviews this week, judge Claudio Pratillo Hellmann made clear that the verdict did not mean that he knew what actually happened in the flat in Via della Pergola, Perugia, on the night of 1 November 2007, when Ms Kercher died.

"They are free because they did not commit the crime," he told La Stampa newspaper. "But this is the judicial truth, not the truth of reality, which could be different. Certainly, Rudy" – Rudy Guede, the only one of the three convicted of the murder who is still in jail, and who left many traces of his presence at the crime scene – "knows what happened and has not said it. Maybe the other two accused of the crime also know, because, I repeat, our verdict of absolution is the outcome of the truth arrived at during the trial. So perhaps they know, too, but we cannot know that."

The DNA evidence

Despite the torrent of claims and counter-claims during the appeal, the verdict hinged on a very simple fact: that the scientists appointed by the court to re-examine the forensic evidence on which Ms Knox and her former boyfriend, Raffaele Sollecito, had been convicted concluded that the DNA on Ms Kercher's bra clasp and the tip of Mr Sollecito's kitchen knife were too weak and ambiguous to be relied on. As Mr Hellmann pointed out, "The law makes it clear that a small doubt, as long as it is reasonable, is enough to absolve." That, and that alone, explains why Ms Knox and Mr Sollecito walked free.

The crime scene and Ms Knox

Yet, if we must accept Mr Hellmann's insistence that we cannot really know what happened, we can at least re-construct the steps by which Ms Knox and Mr Sollecito found themselves in the frame – because those steps, too, were the product of prejudice and imagination, not knowledge.

On the face of it, they were the least likely people to be suspected of involvement. It was Mr Sollecito who called the police from Ms Knox's flat when the two of them found Ms Kercher's bedroom door locked, drops of blood and a broken window; both were present when police battered the door open, and they fled outside in shock and horror. Ms Kercher's British girlfriends flew home soon after their friend's dead body was discovered; but Ms Knox, who could have flown to Berlin to stay with her uncle, insisted on staying in Perugia to help the police.

The prosecutor's obsessions

It would take a devious mind to see in these responses the minds of two guilty people – but the prosecutor on duty in Perugia on 2 November, Giuliano Mignini, was already renowned for the deviousness of his ideas.

Prosecutors in Italy enjoy a degree of power and discretion unthinkable in Britain. Mr Mignini, the father of three young girls, a portly, avuncular figure in his robes, was already well known in Perugia for his conservative Catholic faith, and the ferocity with which he descended on people he regarded as embodying decadent, godless modern behaviour.

He prosecuted an English couple because the garden of their Umbrian cottage had wild poppies growing in it – he claimed they were growing them for the opium seeds. He arrested a foreign stripper in a local club for going too far. He reopened a notorious serial murder case in Florence, not on the basis of new evidence but on the theory that the crimes were committed on behalf of deviant freemasons, a theory inspired by a spiritualist blogger in Rome who said she received information from a long-dead priest. The same blogger, Gabriella Carlizzi, now herself deceased, was the first person to float the idea that Ms Kercher may have died at the culmination of a satanic rite involving Ms Knox and Mr Sollecito.

To some people, the behaviour of Ms Knox and Mr Sollecito was that of two nicely brought-up middle-class kids who had never tangled with the law and still believed that policemen could be trusted to protect you. But Mr Mignini didn't see them like that. He saw a sexy, promiscuous, drug-taking foreign vamp, canoodling with her new Italian boyfriend, showing no conventional signs of grief, identified by Ms Kercher's friends as having had a difficult relationship with the victim, and sitting on her boyfriend's lap in the police station. When the police investigation revealed the hair of a black person in the victim's hand, and it was revealed that Ms Knox had sent a text to her part-time employer, the Congolese bar owner Patrick Lumumba, saying "see you later" – interpreted by the police as an assignation – Mr Mignini had his eureka moment. Caso chiuso, said the police chief. Case closed.

Enter Rudy Guede

It was only in the days following the arrest of Ms Knox, Mr Sollecito and Mr Lumumba that it emerged that the forensic traces in Ms Kercher's bedroom – plentiful traces – were linked not to any of the three but to a fourth person, a local drifter and drug dealer called Rudy Guede.

Now serving a reduced, 16-year sentence for his role, Guede was a sad figure, abandoned by his father, an immigrant from the Ivory Coast, and the wealthy Perugia family that had unofficially adopted him, with little education, no regular work, and no home. Homelessness, it seems, had begun to obsess him: when he had come to a party at the flat below the one shared by Ms Knox and Ms Kercher, he had left his faeces in the toilet. In the weeks before the murder he had broken into a nursery school, a lawyer's office and a flat, in each case making himself at home, turning up the heat, cooking a meal, stealing things. He was questioned by police but never arrested. The reason, the rumour goes, is because he was a police informer.

Guede, who also left his faeces in the toilet of Ms Kercher's flat and who fled to Germany after the murder, was the obvious suspect. He was a loner: there was no reason to suppose others were involved. But, by this time, Mr Mignini had convinced himself and much of the world that this was an exotic and bizarre case, not a squalid rape-murder committed by a disturbed homeless immigrant. And once it had taken root, the idea was too fascinating to be abandoned.

The 'Third Man'

Mr Hellmann's comment that Guede "knows what happened and has not said it" refers to the fact that he has changed his story several times since being named as a suspect. In his first phone conversation with a friend while still in Germany, recorded by police, he said another man was the killer and that he had tried to staunch Ms Kercher's wounds. Later, after learning Ms Knox and Mr Sollecito had been accused, he claimed that he had heard Ms Knox and Ms Kercher arguing before the latter was killed, and that an unidentified man in the house "tried to strike me". But as Ms Knox's lawyer Carlo della Vedova said, "Guede is not reliable – he is a liar." And his conflicting claims are undermined by the fact that nobody's traces but his own were found in Ms Kercher's room.

Tomorrow, Amanda Knox will mark a week of freedom. There are those who continue to paint her, if not as some Jezebel, then as an unfeeling woman anticipating the paydays ahead. It is a curious way to think of someone unjustly imprisoned for four years.