So now we all know the Portuguese word for suspect.

We know where all the roadworks in Praia da Luz are. We've even become experts on DNA. But there's still no answer to the one question we're all asking. Did they, could they, do it?

This column won't primarily be about the guilt or innocence of Kate and Gerry McCann. But I will state straight away that I think they are innocent and are victims of a witchhunt.

However, and you might think this sounds shocking, in practical terms, guilt and innocence are secondary questions. That is because any case against them must be so intrinsically flawed because of police incompetence that it would be impossible to secure a conviction.

The McCann case is without precedent. It has involved two processes of inquiry taking place in parallel and in public: one by the media, the other by the police. One has been characterised by diligence and persistence. The other has involved a mixture of secrecy, smears and unprofessional conduct. Sadly, the latter is the description of the police activity.

Two basic rules govern investigations of any serious crime. One is that the first hours are vital in securing leads and evidence. The other is that the investigation focuses initially on the scene of the crime and then radiates outwards.

In Portugal, the first critical hours were wasted by an uncoordinated and inadequate response by the authorities. It was nearly 24 hours before border guards were alerted to the potential kidnap and a week before experts in child abduction cases were called in. The second rule appears to have been set on its head with an apparent failure to ensure the forensic integrity of the crime scene and desultory and delayed inquiries in the immediate vicinity.

The police's default position when faced with criticism has been to invoke the secrecy that is paramount in Portuguese law.

This was the secrecy, remember, that they said prevented them from issuing a full description of Madeleine. It has not, of course, precluded a steady drip-feed of leaks and rumours that have been specifically aimed at the couple's credibility. I would suggest you don't need a background in detection to know where those rumours came from, nor why the McCanns were designated suspects three days before they were to leave the country.

I write this as someone who has experience of police work. I have also been at the centre of an investigation - albeit of a very different kind - that involved intense media interest, massive amounts of speculation, but ultimately no evidence to support what might be termed the case for the prosecution.

I know, therefore, the intense pressure that the police face from both the establishment and the media and the temptation to lay down a smokescreen to cover up their own failings. I believe that is what we are seeing in the case now being constructed against the McCanns.

The McCanns have become the authorities' worst nightmare - a permanent and vocal reminder that they got things badly wrong, a standing rebuke to their failings.

That has led to something we've never seen before. Up to now, miscarriages of justice have always been discovered after the event. New evidence is found, or existing facts re-interpreted in the light of scientific progress. Here, I believe, we are seeing for the first time a miscarriage of justice unfolding in real time before our eyes.

There is nothing we can do to stop it. That is a matter for the family's lawyers. But it has made it impossible for anyone to sit on the fence. I hope we can make our position clear by refusing to join in the 'smoke without fire' speculation that has filled the vacuum where facts should be in the past few days.