In 1995, when two eight-year-old girls were kidnapped, Dutroux, a convicted sex offender, was a prime suspect from the start, yet he wasn't arrested for 14 months. By that time, four of his captives - including the two girls - were dead. Since his arrest, 20 potential witnesses connected with the case have died in mysterious circumstances, fuelling suspicions of a cover-up reaching the highest levels.

I have spent the last six months making a documentary about the investigation. Early on, I was told by one senior government adviser: 'You must not underestimate the terrible record of our Belgian justice system.'

It's a system which today appears paralysed, unable to prosecute the accused, his wife and an alleged accomplice. With each successive year in jail without trial their case against the Belgian authorities for a breach of human rights grows stronger. The official explanation for the delay is that hysterical conspiracy theories forced investigators to search for paedophile networks which didn't exist. But far from being investigated, leads pointing to a network seem rather to have been ignored or buried.

Dutroux's wife, Michele Martin, a former primary school teacher and the mother of his three children, has admitted that, in 1995, she knew two small girls were incarcerated without food or water in a secret dungeon in the cellar of a house they owned in Charleroi. She told police she visited the house to feed their dogs while her husband was in jail on car-theft charges, but she was 'too frightened' to feed the girls.

Months later Dutroux led police to the emaciated bodies of Julie Lejeune and Melissa Russo, the two eight-year-olds who had been kidnapped more than a year before. They were buried in the garden of another of Dutroux's homes. An accomplice, Michel Lelievre - a drug addict and petty thief - told police soon after his arrest that the girls had been kidnapped to order, for someone else. The chief suspect was Jean Michel Nihoul, a Brussels businessman, pub-owner and familiar face at sex parties. While they had been in prison, Lelievre told police, Dutroux and Nihoul met frequently in the exercise yard, making plans. The judge investigating the case, Jean-Marc Connerotte, believed Nihoul was the brains behind the operation. But, as the network began to unravel, Lelievre suddenly stopped co-operating, saying he had been threatened.

I met Nihoul in a restaurant in Brussels. 'I am the Monster of Belgium,' he roared at me by way of greeting. He is confident he will never come to trial and that the evidence against him will never be heard by any jury. During the course of our meal he, apparently playfully, grabbed me, tickling, and finally pulled me over on to him in the restaurant booth until I had to appeal to my colleagues for rescue.

He will never come to court, he said, because the information he has about important people in Belgium would bring the government down. The Monster of Belgium denies he's a paedophile but seemed to enjoy his notoriety and demanded £1,000 for his story. We declined his offer. Every documentary likes a monster but we don't pay for interviews and frankly I'd already had enough.

But we did need to offer Nihoul a right to reply to the accusations made by Regina Louf, a woman now aged 33 whose testimony has divided Belgium. Louf came forward after Judge Connerotte made an appeal to victims of paedophiles to tell police what they knew. Connerotte, the man who had arrested Dutroux and saved two teenage girls from his dungeon, was a hero in Belgium. Louf was the first of 10 to come forward.

She told investigators how from the age of 12 she'd been 'given' by her parents to a family friend, Tony Van den Bogaert, who'd had a key to their house. He would collect her from school and take her away for weekends to sex parties where she was 'given' to other men and secretly filmed having sex with them. 'It was highly organised,' she says. 'Big business. Blackmail. There was a lot of money involved.'

In 1996 she related her experiences to a police team under carefully filmed and supervised conditions. She described certain regular clients including judges, one of the country's most powerful politicians (now dead) and a prominent banker. She gave the police the names by which she knew these men, detailed the houses, apartments and districts where she'd been taken with other children to entertain the guests.

This 'entertainment' was not just sex, she told the police. It involved sadism, torture and even murder, and again she described the places, the victims and the ways they were killed. One of the regular organisers of these parties, she claimed, was the man she knew as 'Mich', Jean Michel Nihoul, 'a very cruel man. He abused children in a very sadistic way', she said. Also there, she said, was the young Dutroux.

'Dutroux was a boy who brought drugs, cocaine to these parties - he brought some girls, watched girls. At these events Nihoul was a sort of party beast while Dutroux was more on the side.'

Louf's testimony was vitally important. If true, it placed Dutroux and Nihoul, suspected accomplices in the latest child abductions, together at the scene of similar crimes 10 years before. Police began to check her story. But then something changed.

In October 1996, Connerotte, the only man who has ever advanced the Dutroux investigation, was sacked from the case. He had attended a fund-raising dinner in support of the victims' families and was accused of a conflict of interests.

A crowd of 400,000 marched on the Palace of Justice in Brussels to protest. The father of one of the murdered children, Gino Russo, spoke for the demonstrators. 'It was like spitting on the grave of Julie and Melissa,' he said.

Connerotte was replaced by Judge Jacques Langlois, for whom this case would be his first assignment. Langlois has spent the last five years in constant conflict with the public prosecutor assigned with him to the case, Michel Bourlet. Since Connerotte was sacked, according to the Russos, the Dutroux file has acquired no new evidence.

Next to be dismissed, a few months later, was the special team of police officers who had interviewed Louf and the other witnesses. By now the police believed they had verified key elements of Louf's story. At least one of the murders she described matched an unsolved case. One of the police officers in the team, Rudi Hoskens, had been assigned to re-examine that case and was convinced she had witnessed the murder: 'She gave us some details that made us think it's impossible to give without having been there at that place - the way the body was found at that time, and the way she described the person who was killed.'

What Louf had described was a macabre torture which had eventually killed a 15-year-old girl she knew as Chrissie. 'It was a sort of bondage,' she told me, 'so her legs and her hands and her throat were connected with the same rope, and so when she moved she strangled herself.' Louf insists both Nihoul and Dutroux were there that night. Nihoul, she claims, took part in the murder, a charge he denies. Dutroux, she says, watched.

Christine Van Hees's body had been found in 1984 dumped in the grounds of a disused mushroom farm on the outskirts of Brussels. The farm was later demolished but in 1996 Louf described to the police team its intricate details, the wallpaper, the sinks, hooks on the ceiling, a network of stairs and adjoining rooms unique to that building.

When I put this evidence to Anne Thily, Prosecutor General of Liège, in overall charge of the Dutroux affair, she gave me a shrug and repeated the official line in Belgium, that Louf is a fantasist and has invented everything.

This is not the view of the man who grew up at the farm, the son of the former owner, who showed me photographs of the house and the mushroom factory. He said: 'I have never met Regina Louf. All I know is that she could not have described the house as well as she did unless she'd been there. It was two houses joined together in a strange way. It would be impossible to invent it.'

For 12 years the unsolved murder of Van Hees gathered dust in the Brussels files under the direction of Judge Van Espen. Two years ago a Belgian journalist revealed the close relationship between Judge Van Espen and Nihoul and his then wife.

As a lawyer, Van Espen had repre sented Nihoul's wife. Van Espen's sister was the godmother of Nihoul's child. Yet, when Louf accused these two of the murder, Judge Van Espen saw no conflict of interest, no reason to resign. Nor was he sacked, as Connerotte had been. Instead he was allowed to order the police officers to stay out of the case. Van Espen only resigned as the judge in charge of the mushroom factory investigation in early 1998 after his relationship with Nihoul was exposed.

In the spring of 1997 Louf's interrogators had been sent home without explanation and a new team was assigned to 'reread' her testimony. The press was briefed that the previous team had been removed because they had manipulated the evidence of Louf, who was then known by the code name X1. It is a charge which the police team has always vigorously denied and which has never been substantiated.

And then the media campaign began. Louf's name was leaked to the press. The government-owned TV station RTBF began a campaign designed to prove that Dutroux was an 'isolated pervert' kidnapping girls for himself, that there was no network, that Nihoul was innocent and Louf was a liar.

Belgium's flagship current affairs television programme, Au Nom de La Loi , floated Louf's face over a backdrop of crows pecking over debris orchestrated by a Blair Witch-style soundtrack. Her ageing parents appeared as tragic victims of a deranged fantasist, whose false memories had blighted their last years.

What the programme makers knew but didn't say was that the parents had already admitted to police that a family friend in his forties, Tony Van den Bogaert, had had a key to their home and unlimited access to their 12-year-old daughter. Nor did they tell their viewers that Van den Bogaert had himself admitted his relationship with Louf to police. Van den Bogaert lives freely on the borders of Belgium and Holland unmolested by the law or the press. Au Nom de La Loi has never attempted to track him down and expose this self-confessed paedophile. Instead they have devoted hours of air-time to destroying the name of his victim, Louf, whose only offence appears to be that she was prepared to testify about the organised abuse she'd suffered as a child.

This campaign has succeeded. Judges have announced that Louf will not be called as a witness in any future trial of Dutroux or his associates. Her testimony and that of all the 10 witnesses who came forward to Judge Connerotte has been declared worthless.

Noone has ever followed the Dutroux investigation more closely than Gino and Carine Russo, the parents of Melissa. What alarms them more than anything is the dearth of evidence or independent witnesses in the whole affair.

The Russos have access to the dossier of evidence which will, eventually, be presented to a jury. What alarms them, they say, is that it contains little more than the highly suspect version of events offered by Dutroux and his wife. This is crucial because while Dutroux admits incarcerating their daughter in his home, he denies her kidnap, rape or murder. Dutroux even claims he tried in vain to save the girls and that Melissa died in his arms.

The Russos have lived this nightmare ever since Melissa disappeared with her friend Julie in June 1995. Although Dutroux was a known paedophile, police didn't search his house for five months, and when they did they failed to find the girls, despite the sound of children in the cellar.

When a parliamentary commission examined the series of failures in the Dutroux investigation the police officer responsible, René Michaux, claimed it was a genuine mistake, that the entrance to the dungeon was well hidden and that the children's voices seemed to come from outside. He found a speculum on the floor which he lifted, handled and returned to Dutroux's wife without forensic analysis.

They found films which went undeveloped and videos which they didn't watch. Had they done so, they would have seen Dutroux building the dungeon. Instead Dutroux continued to abduct girls. In August 1996, four days after his last kidnap, he was arrested. He showed police the dungeon from where two girls were freed and then he led them to where Melissa and Julie were buried.

Carine Russo was not allowed to see her daughter's body. 'I begged and pleaded. I went with my lawyer but they refused. They told me the law did not permit it. "But who will identify my daughter?" I asked them. "Who will confirm that it's her?" "Dutroux has identified her," they told me.' Then Carine looks at me. 'It is stupefying,' she says.

The autopsy report reveals Melissa was raped repeatedly over a prolonged period. But there is nothing, no DNA evidence, no witness sightings, no forensics of any kind to show whether it was Dutroux, or anyone else.

Carine Russo points to a wall of files in her office. 'Where are the results of the swabs taken from Melissa's body for analysis? We know swabs were taken. It says so in the reports. But there are no results. I've asked the prosecutor repeatedly and no one seems to know.'

After their years of grief and their betrayal by the Belgian police and judiciary, the Russos barely believe a word of the official version: that Dutroux, the lone paedophile, kidnapped the girls for his personal use and kept them in the cage in his cellar until their death of starvation the day he returned home after four months in jail. How, they ask, could two children survive alone with virtually no food or water for four months?

The Russos suspect the girls weren't there at all. A number of reported sightings of Melissa, one in an upstairs room of a Charleroi nightclub, which were never followed up, have convinced them that someone else had access to the girls while Dutroux was in jail. Why else, they ask, were the hairs which detectives gathered from the dungeon in Dutroux's cellar never sent for DNA analysis? Why did Judge Langlois, Connerotte's replacement, refuse to have them tested despite pressure from his prosecutor, Michel Bourlet, who believed that a DNA identification of those hairs might reveal who else was involved.

Langlois's boss, the Prosecutor General of Liege, Anne Thily, says: 'There was no need to get the hairs analysed as no one else entered the cage. There was no network so there was no need to look for evidence of one.

'In any case,' she continued, 'the hairs have all now been analysed - all 5,000.' And the results of this analysis? 'Nothing.' Thily flashed me a triumphant smile. 'No evidence of any relevance in the Dutroux affair. Which proves, of course, that Langlois was right all along.'

But this is not true. Sources central to the investigation confirm that to date the hairs have still not been analysed. How can such a senior figure lie so brazenly? Another Belgian mystery.

'Who raped the children?' I asked Thily. 'Dutroux of course.'

'But he denies it. How will you prove it to the jury? There was no DNA test?' Now she was indignant. 'There were DNA tests, Madame.'

'And the results?' 'Inconclusive. The bodies were too decomposed to test for DNA,' she says.

But this too makes no sense. The autopsy states clearly that the bodies were not decomposed. Samples were taken. But no one seems to know what has happened to the results.

Bruno Tagliaferro was someone who knew, or claimed to know, about the abduction of Julie and Melissa and the car which was used. The Charleroi scrap metal merchant told his wife in 1995 that Dutroux was trying to get him killed. It was something to do with the car in which girls had been taken.

When he was found dead, apparently of a heart attack, his wife Fabienne Jaupart, refused to accept the verdict. Samples of his body sent to the US for analysis showed he'd been poisoned. Jaupart told reporters she was determined to find her husband's killer, but soon she too was found dead in her bed, her mattress smouldering. It was declared suicide. Since 1995, there have been 20 unexplained deaths of potential witnesses connected with Dutroux.

'In Belgium,' says Regina Louf smiling, 'if you're a potential witness you're either dead, or like me, mad.'