Kate McCann remains the key in the hunt to find her daughter, Portuguese police said as they began work on a minute-by-minute reconstruction of her movements.

Detectives want to double-check her every step on the night Madeleine disappeared in Praia da Luz, the Portuguese newspaper 24 Horas said.

Police are interested in an alleged 90-minute gap when they believe Mrs McCann was alone with her children in the holiday apartment.

Friends of the McCanns said they had not been contacted to give any further information about the night of May 3. The focus on Mrs McCann comes as an assessment by five British criminal investigation experts raised unsettling new questions about the case. Here, we present their findings:

It is 168 days since Madeleine McCann vanished, and the fog of conflicting conspiracy theories continues to grow.

Will we ever find Maddy? Expert Dave Canter says she was most likely abducted

Only one fact remains undisputed: the alarm was raised by Kate McCann just after 10pm on May 3. Beyond that, it is impossible to tell fact from fiction.

Uncertainty reigns because we know so little: the Portuguese secrecy laws forbid the police and the official suspects - Kate and Gerry McCann and Robert Murat - from talking about the case.

Speculation based on unofficial leaks is all the media have to go on.

Unfounded and uncorroborated, too many of these rumours are as far-fetched as they are malicious.

So to help provide some firmer ground amid the conflicting reports, Channel 4's Dispatches sent five British experts in criminal investigation to Praia da Luz to assess the state of the inquiry. Between them, they have 134 years of experience in dealing with serious crimes, such as abduction, murder and paedophilia.

Their aim was to shed light on each area of the case that was controversial and confusing: the handling of forensic evidence, the search for a likely abductor and the possibility that the family might be involved in Madeleine's disappearance.

They visited the key locations, reviewed all the evidence in the public domain and drew on their previous cases as well as statistical probabilities to assess what might have taken place that night.

The experts give their view: Search expert Gary Lig (left) asks whether the Portuguese police searched the sewers in Praia da Luz, while media handler Matt Tapp says Portuguese secrecy laws hampered the investigation

Mistakes: Experts (from left to right) David Canter, David Barclay and Chris Stevenson explain where the Portuguese police have been going wrong

Their work threw up revelations that are as intriguing as they are disturbing.

The first stage of their investigation began with a review of the three basic theories about how Madeleine could have disappeared. First: could she have wandered off by herself? Second: was she abducted? And last: could something have happened to Madeleine in the apartment itself?

The first theory is perhaps the simplest to deal with, say our experts - and the easiest to dispel.

Professor David Canter, Director of Investigative Psychology at Liverpool University, is one of Britain's foremost behavioural profilers. After visiting the McCanns' holiday complex, he is insistent that if Madeleine had woken up and wandered outside, she would not have gone far.

The apartment patio has steps down to the road, but Madeleine would have been used to walking towards the pool, where her parents were dining.

To get there, she'd have been in an area lit by street lights, and would have been drawn to the welcoming reception area.

If that had been the case, she was bound to have been spotted and rescued by one of the adults out and about that night.

Lost or abducted? Madeleine wouldn't have walked away alone without someone seeing her, our experts say

The second theory - that Madeleine was abducted - is more complex.

Her parents immediately reported that she had been snatched. Kate is quoted as saying 'They've taken her. She's gone' the moment she realised that Madeleine was missing.

Two other witnesses reported Gerry as saying on the night that an intruder had broken in through the shutters in the children's bedroom.

Our experts were more cautious, pointing out that no one has yet asked why Madeleine had been targeted.

They were troubled by what kind of person would have selected a four-year-old girl, asleep next to her infant twin siblings.

David Canter reviewed the range of possibilities - from a local oddball, to an organised gang of traffickers or paedophiles.

'If we think about an organised criminal network abducting children to traffic them in some way, then they would not have gone to an area such as Praia da Luz,' he concluded. 'There are so many adults around.'

Mark Warner, which operates the Ocean Club, had a creche and nannies, and there were too many adults working or staying at the complex.

'Unfortunately, there are all sorts of opportunities to abduct children from Eastern Europe,' said Canter.

'If a gang wanted a blonde child, like Madeleine, then the orphanages in Croatia are full of them.'

So who would have taken Madeleine, and ignored her brother and sister? A paedophile, perhaps?

Our team included Professor Dave Barclay, a leading forensic scientist who has worked on 215 'cold case' murders in the past six years. In his view, the layout of the complex made it 'a pervert's paradise'.

Praia da Luz: Prof Dave Barclay described the complex where the McCanns were staying as a 'pervert's paradise'

Some of the apartments overlook the pool and give an unobserved vantage point to anyone wanting to watch children by the pool.

Portugal has many more paedophiles than it cares to admit, and an odd attitude towards underage sex. Under Portuguese law, the age of consent (between couples of the same age) is 13.

Our experts also noted that when children are found to have been abused, the victim is taken into care, while the abuser is often left free.

Not before time, Portuguese police last week raided 80 people's homes and seized computers, hoping to find pictures of Madeleine on their hard disks. It's been another dead end.

So if it wasn't a paedophile or child-trafficker, who could have taken her?

Prof David Canter applied his experience to come up with more refined possibilities about likely abductors.

'A mother who is very disturbed and trying to abduct a child to replace her own dead baby wouldn't have gone for Madeleine. She would have gone for one of the twins who, being that much younger, could have been absorbed into her family.'

Moreover, he feels that four years old is an odd age for any child victim to be abducted - too young to be of interest to most paedophiles, too old to be trafficked or stolen as a baby-substitute.

But then if Madeleine did not wander off, and was not abducted, that leaves only the third possible scenario: that her parents were involved in her disappearance.

In September, that is what Portuguese police started to believe is a serious possibility.

Suspects: This picture was taken only 11 days into the investigation when Maddy's parents were still witnesses. It was not until months later that they became suspects. Soham detective Chris Stevenson says 'you have to look at the family first'

The statistics are compelling. Chris Stevenson was the Detective Chief Superintendent who caught Ian Huntley in the hunt for the murderer of Holly Wells and Jessica Chapman, and was our team leader.

'You have to look at the family first,' he says. 'Three out of four child deaths are at the hands of their family.'

In Britain, he says, the family would have been the very first to be interviewed, and the crime scene combed for evidence to corroborate their story.

Yet in Madeleine's investigation, the forensics have apparently become central to the case only belatedly.

Our experts believe that if this is the case, it raises serious questions about the Portuguese police's priorities.

Prof Dave Barclay, our forensics expert, feels strongly that the apartment should have been sealed off immediately when Madeleine was reported missing, and subjected to proper combing for evidence.

Moreover, he regards the speculation about the DNA results that have been found in the apartment and the McCanns' hire car as 'interesting', but potentially no more than that.

As Madeleine was known to have cut her knee during the holiday, it would not be unusual to find traces of her DNA in the apartment and on her clothing, which could have been transferred onto other items.

'Forensic science is not just about a single test result,' says Barclay. 'It's about setting it in context.'

He is particularly sceptical about the reported presence of bodily fluids found in the McCanns' hire car.

'You've got to work out where the body has been and how it got transported in the car that wasn't hired until 25 days after Madeleine disappeared,' he says.

'It's almost incomprehensible. So we should just wait and see what the results show before leaping to any conclusions. It's not beyond the bounds of possibility that they will exonerate the McCanns.'

Indeed, the whole way the investigation has been run by Portuguese police is of deep concern for our experts.

The time immediately following a child going missing is known as the 'Golden Hour'. It's when the victim is most likely to be found alive.

As the hours pass, that chance diminishes. Above all, search parties need to be carefully briefed and organised with military precision. In Madeleine's case, there are grave doubts among our panel of experts that this is what took place.

As soon as Kate McCann raised the alarm, at 10.10pm, Mark Warner staff fanned out across the resort, and local residents volunteered to explore the surrounding streets and houses, and then the nearby beach.

But how well managed was it? What skills would be needed to find her, if she was hidden?

Search expert Gary Lig, previously from West Yorkshire Police, has advised on search plans in more than 100 murders and disappearances. He argues: 'Even trained police officers may miss crucial signs. I know a case where they searched a house three or four times before finding a body.'

Visiting Praia da Luz for our review, he was particularly concerned to discover, from local maps, that a network of giant sewers runs under the town. Were they searched? If so, how thoroughly?

The Portuguese police declined to respond to our questions, but we were told the volunteers were guiding themselves much of the time that night - and for several days after.

Even more worryingly, this hit-or-miss approach appears to have applied to the giant rubbish bins scattered around Praia da Luz.

Charlotte Pennington, a Mark Warner nanny, says: 'We were told to search everywhere, including the bins. I saw the police searching, but didn't see them looking in the bins. I don't think we looked in every bin.'

Incredibly, even with an active hunt for a missing child going on, we discovered that the bins were emptied in their usual fashion. Moreover, no one seems to have checked the landfill site to which they were taken.

For Professor Barclay, this represents a major oversight of a potential hiding place - if not for Madeleine's body, then for vital evidence such as the pyjamas she was wearing.

'What are you going to do with the body of a four-year-old?' he asks.

'You can take it somewhere in a car, but you risk being stopped and caught with a body. Or you can dispose of it locally - in one of the bins, in which case it's taken away and never found.'

Detective Chief Superintendent Stevenson agrees that the bins could have yielded vital clues.

'Ideally you would secure all the bins in the area and make sure the local authority don't dispose of the contents until the search team has gone through them all.'

The search for Madeleine was further hampered by the fact that the Portuguese police were unprepared both for the need to act quickly and the media firestorm that followed.

They've had only two cases of child abduction in 15 years.

Meanwhile, the British police have had so many cases that they have learned painfully from their mistakes, and have updated their training manuals and protocols.

In Britain if such an alarm were raised, they would swing into action at once - with search teams, an incident room, media notification of key identifying details, border and airport controls and so on.

But in Portugal, the search for Madeleine took place amid an information vacuum that, said our experts, would have significantly decreased the chances of locating Madeleine within those first few days.

'When a crime has happened, you need as much accurate intelligence as possible. We're used to securing that information partly through media appeals,' explains our final expert, police media specialist Matt Tapp, who handled the media for the Soham murder inquiry.

'None of that happened with Madeleine because the police say their laws of secrecy meant they were legally bound to say nothing.'

He is impatient about the conflicting stories that are still published daily. 'It's what you can expect when police choose or are not allowed to fill the void. Others fill it in their place.'

In the meantime, the hunt for Madeleine goes on, with each day bringing fresh claims.

Was Madeleine abducted? Did her parents kill her? There is an even more haunting possibility: that the investigation has been so poorly handled that we may never know what took place that night.

'I still feel abduction is the most likely possibility,' concludes forensic psychologist David Canter.

But he's pessimistic about finding the truth amid all the confusion.

'The trails may have gone so cold over the passage of time that even finding a body would not explain what really happened. We may still be talking about this case in 20 or even 50 years' time.'

And still wondering: What happened to Madeleine?

? Roger Graef is executive producer of Searching For Madeleine: A Dispatches Special on Channel 4 Thursday night at 9pm.