As a criminologist, former detective and authority on paedophile crimes, I have followed the Madeleine McCann case from the very beginning.

What happened to Madeleine?

What happened to her on that fateful night in May 2007? Where is she now, is she alive or dead? Was she murdered in her parent's apartment, and if so by whom?

Was she abducted from where she slept in between her brother and sister who remained asleep throughout?

Or did she wander out of the apartment looking for her parents and become the victim of an opportunistic predatory paedophile, who either used her for his own sexual gratification or smuggled her out of the country?

With apparent sightings in Morocco, Spain, France and the Netherlands you could easily believe the latter.

The most sinister of all these questions was whether Gerry and Kate McCann were involved with the disappearance of their own daughter.

In order to establish answers it is necessary to examine the police investigation.

The detailed examination of a scene plays a crucial part in any major investigation and after a critical incident like this, all police officers know it is vital to preserve the crime scene, build the foundations, complete forensics and identify possible witnesses immediately.

But I saw no evidence of this being done and it was without any doubt, the worst-preserved crime scene I have ever witnessed.

This investigation is like no other I have seen before or will hopefully ever see again.

For the past year the Portuguese police have had two lines of inquiry: the first, that Gerry and Kate were involved in Madeleine's disappearance and the second, that a stranger abducted her.

However, they have focused steadfastly on their first line of enquiry.

The public prosecutor's office in the Algarve believes police should keep looking for evidence against the McCanns even though doubt is being raised from other quarters, with officials in the Portuguese government asking for their arguido status to be lifted.

The investigation is all but closed and recent interviews with the 'Tapas Seven' revealed no new leads or discrepancies but did allow the police to establish the exact movements of Gerry and Kate both on the evening Madeleine disappeared and on the previous evening.

The next major development will be if Portuguese police manage to stage a reconstruction so they can examine the behaviour of all parties concerned - but this would put Gerry and Kate back under Portuguese jurisdiction.

It is quite correct for the police to consider the parents as suspects but what evidence do they have against them?

The time line is crucial. The last time Madeleine was seen by anyone independently was around 6.30pm with the alarm raised at 10pm, thus establishing a window of opportunity of approximately three-and-a-half hours.

However, both Gerry and Kate were with their friends in the tapas bar from approximately 8.30pm, only ever being away from the table for short periods of time.

So one or more of the parents did have the opportunity - but what would be the motive? Could it have been an accident or a deliberate act?

I don't believe for one minute it would have been deliberate so therefore could she have suffered an accident, causing the parents to panic?

At this point they could easily have explained away an accident.

But, if the theory is to be believed, they would have had to cold-bloodedly dispose of her body and then return to have dinner with their friends.

It is possible as most murdered children are killed by a relative or someone they know. The police are working on this premise although in this particular case this is where the theory falls down.

The police believe they hid Madeleine's body and didn't return to the location until five weeks later where - under the watchful glare of the world's media - they placed her by now decomposed body into the rear of their Renault Megane hire car before completely disposing of it.

The police have come to this conclusion because they have found partial DNA in the boot of the hire car, which I believe has either come from transfer when they moved Madeleine's clothes and belongings or even from those of her brother or sister.

While trafficking of children is a far greater problem than is acknowledged, Madeleine does not fit the profile.

Mark Williams-Thomas

Girls are trafficked for two reasons: into the sex trade as prostitutes or for domestic slavery, both with financial gain to the seller and purchaser. Who was to gain from her trafficking?

I believe the so-called sightings in other countries, although genuinely intended, are a distraction and prove to be of little value after the initial two weeks. Who is going to openly walk out with the most wanted child in the world?

I believe what happened on May 3 was that Madeleine woke up, cried for a short while, realised her parents weren't in the apartment as they had not come in to settle her, so she climbed out of bed and walked around the apartment.

She found the back patio door was insecure and partly open so she walked out, went down the small flight of steps and out of the gate, turning right down towards the entrance to the resort and the Tapas bar.

It was at this point that she was most likely abducted by an opportunistic predatory paedophile.

Interestingly the police dogs first tracked a scent down this exact route. Unfortunately, although it is within range of the supermarket CCTV it was not working.

I don't believe a paedophile was watching the apartment nor do I believe an offender entered the apartment - this would be too high-risk.

I accept that statistically the abduction of a child is very rare, with approximately six children abducted every year in the UK, but if we look at the abduction and murder of Sarah Payne we can see how my theory stands up.

On July 1, 2000, Sarah was playing a game of hide and seek in a cornfield with her sister and two brothers, received a knock on the head and decided to walk back to her grandparents' home.

She walked through a gap in the hedge with her brothers' only seconds behind - but by the time they reached the lane, Sarah had disappeared.

One of the brothers saw a white van coming down the road towards him, its wheels spinning. The driver seemed in a hurry to get away. Unknown to them, the man behind the wheel had just abducted Sarah. She was taken in a matter of seconds.

This is why I believe even with such a small window of opportunity - the distance between the back entrance to the apartment and the 40 yard walk to the Tapas bar - that Madeleine became the victim of an opportunist and predatory paedophile who either lived in or had direct contacts with Praia da Luz.

This is not a resort that you would just happen upon.