At first he just toyed with the idea of killing

When finally they caught him, it was a fluke. He had parked his lorry outside a football stadium in a small town in north-eastern Spain and he was waiting for dark to get rid of the body of his latest victim. By sheer chance, a technician was installing a CCTV camera on the wall of a neighbouring factory and, while he was adjusting it to focus on the factory gate, he accidentally panned across the stadium car park and picked out the lone lorry, with its owner's logo splashed across its flank.

The following day, the Spanish police found the body of the murdered woman; collected and checked any local CCTV; found the accidental footage; traced the lorry to Germany and asked German police to pick up the driver, perhaps as a witness, perhaps as a murderer. So it was that while he was working in Cologne, on November 17 2006, Volker Eckert, aged 47, finally came to the end of his serial killing.

For an hour or so after he was arrested, he shrugged at police and professed his total ignorance of the body in the Spanish car park. Then he did a strangely revealing thing. He told the police he had a headache and that he needed special medicine from the cab of his lorry. An officer went to fetch it and noticed three Polaroid photographs lying between the two front seats. Each showed a dead woman with a noose around her neck. Next to the bunk at the back of the cab, he found handwritten notes describing other murders, and two lengths of rope.

Eckert was held and soon confessed to killing six women. As European police began to reconstruct his life, they found the clues to other crimes - in Spain, Germany, Italy, France and the Czech Republic. Murders and attempted murders, more than 50 of them, stretching all the way back to his adolescence when, as a 14-year-old schoolboy, he had called on a girl from his class to ask her about homework and strangled her.

Volker Eckert was no kind of master criminal. He was an uneducated and unskilled man of average intelligence who had earned a living first as a house painter and cleaner, then as a long-distance lorry driver. Yet for years he had been able to cruise around Europe getting away with murder.

He succeeded for two reasons: he killed in different countries while the police remained trapped inside their national borders, failing to consult each other; and he killed disregarded people. With the one exception of his 14-year-old classmate, all the women he killed were poor, or migrants, or prostitutes. With rare exceptions, they vanished, and nobody asked why; when their bodies turned up, nobody came to claim them; they were buried, and nobody mourned them or demanded the culprit be found.

And now, even after his capture and confession, even after the police have traced his involvement in dozens of attacks, there has been no proper reckoning and the questions raised remain unanswered. This is a story about Europe, about open borders and a dark underworld of hidden lives.

It begins in innocence. When he was caught, Eckert told a psychiatrist from Munich, Dr Norbert Nedopil, how, as a child growing up in a small town in East Germany called Plauen, he had sometimes liked to play with a doll that belonged to his younger sister, Sabine. He treated it well. He especially liked the doll's hair. It was long, and he fondled it. He was only nine or 10 but he had reached puberty early and, one day, kneeling beside his bed, stroking the hair of the doll, he started rubbing himself against the side of the mattress and had an orgasm. He liked it, so he did it more often.

Soon after that, in the attic of his family home, he found a hairpiece that belonged to his mother. He liked that, too. The more he used the doll and the hairpiece, the more he fastened on hair as the object of his love. By the time he was 12 or 13, however, he began to tire of artificial hair and, as he told Dr Nedopil, he would sit in class and gaze at the hair of the girl in front, tormented - real hair, long hair. He wanted to touch it as he had touched the doll's hair. He told the psychiatrist that this idea became so overwhelming that there were times when he failed even to eat.

Slowly, the idea of violence formed in his mind: if he was ever to be able to act out his dream, he would have to subdue the girl. For months he worked on the fantasy, now practising strangling the doll before toying with her hair. Perhaps it is significant that when he was 13, in 1973, his parents split up and he went wild with anger, stealing his mother's car and disappearing for several weeks, evading the police in a mad chase before they finally caught up with him and took him back to his unhappy home.

On May 7 1974, two months before his 15th birthday, the boy went up into the attic above his flat. This was part of a communal space that stretched the length of the building. He walked to the far end and took the staircase down into the corridor in front of Silvia Unterdorfel's flat. She was in his class at school. She had long, beautiful hair. She answered the door and let him in and, realising she was alone, Eckert finally lived the fantasy that he had so often rehearsed, gripping her around the neck with both hands, pressing his thumbs on her windpipe, holding on until she collapsed unconscious and he could bury his hands in her hair. At this point, Silvia was still breathing. But the boy worried she would get him into trouble and so he followed his logic and took a clothesline and killed her, tying the loose end to a doorknob to try to make it look like suicide. He went back to his flat, ate well and masturbated with the memory of his deed.

And he got away with it. Even though Silvia's stepfather was a police officer who was sure the girl had not killed herself; even though a stovepipe had been wrenched off the wall at some point in Eckert's attack; even though it was almost impossible to see how the girl could possibly have hanged herself from a doorknob: the file was closed. This was in the old East Germany and, looking back years later, the police who finally investigated Eckert's crimes concluded that perhaps the authorities had preferred not to admit that the stepdaughter of an officer in the Volkspolizei could possibly have become the victim of such a scandalous crime.

This was the beginning of a pattern of official failure, as all the resources of modern Europe were unable to stop Eckert over more than three decades, seeming barely to notice as he left a trail of women, some unconscious, some dead, across the continent.

Most is known about the final six or seven years of his activity. In 1999, at the age of 40, Eckert qualified as a long-distance lorry driver - a job that, he admitted, he chose specifically for the opportunities it would give him to indulge his fantasy. He would set out on trips, planning his attacks with deep excitement, as Dr Nedopil recorded, "like a child looking forward to Christmas". By now, he was addicted not only to women's hair but to the act of strangulation itself.

He confessed that on June 21 2001, travelling back from a job in Spain, he stopped his lorry in the streets near the railway station in Bordeaux, south-west France, and picked up a Nigerian prostitute named Sandra Osifo. She was 21. He chose her for her long hair, which turned out to be a wig. Within an hour, she was dead. Four days later, her body was found in a ditch by the road 90km north, her Spice Girls pendant still lying loose around her throat. Police in Poitiers identified her and then made no more progress in finding her killer.

Two months later, in August 2001, back in Spain, Eckert picked up Isabel Beatriz Díaz in Lloret de Mar, a popular tourist town on the north-east coast. He later confessed to police that, when he started to strangle her in the cab of the lorry, she fought back with great strength and he had sex with her as he killed her. He threw her body from the cab near the motorway junction at Maçanet de la Selva. It was not found for two months. Nobody had reported her missing. Police in north-east Spain made no progress in finding her killer.

Similarly police investigations in France went nowhere after the murder of Benedicta Edwards, aged 23, from Sierra Leone, working as a prostitute in Troyes, who was strangled and dumped naked on a footpath in August 2002. (Eckert never confessed to this, but he was driving in the area at the time and withdrew money with his credit card in Troyes just before she was picked up.) And police in the Czech Republic failed even to identify the body of a woman found strangled and dumped naked by the motorway near Pilsen in June 2003. (Again Eckert did not confess to this, but again he was found to have been in the area at the time, and police now strongly suspect he was responsible.)

By now, clearly, he was not merely strangling his victims into unconsciousness but routinely killing them. He was also using a Polaroid camera to take pictures of their bodies, and cutting off their hair and stealing "trophies" such as clothing or handbags or make-up. And because nobody stepped in to stop him, the murders went on: a Ghanaian woman, Ahhiobe Gali, in north-east Italy in September 2004; Mariy Veselova, a Russian, found dead in Figueras near Gerona in February 2005; a Polish woman, Agneska Bos, in north-eastern France in October 2006; and in November 2006, Miglena Petrova Rahim, aged 20, from Bulgaria, at Sant Julia de Ramis, near Gerona. She was the final victim, when Eckert's lorry was caught on CCTV.

Beyond these nine murders - six confessed and three discovered - the police who finally investigated his crimes found evidence of three others. The notes that the German officer found in Eckert's lorry indicated he had killed an unknown woman in France in February 2005, as well as two others in the mid-1990s in the Czech Republic where he had driven across the border from his small flat in Hof, near Nuremberg, before he had begun his international career.

Searching the Hof flat, police found more photos. Some were pictures of women cut from magazines. Others were real victims, with nooses around their necks and notes about what he had done to them. Some of these women have never been identified. Beneath the bed, they found a lifesize rubber doll decorated with hair and other trophies that he had collected from his victims.

Looking back into his past, police then found a sequence of violence stretching from his murder in 1974 to his lorry-driving phase. And whereas the western European authorities had failed even to notice that there was a pattern of killings that needed to be investigated, the old East German authorities had, at least, managed to arrest him three times, although they had then failed to recognise the threat he posed.

They arrested him first in 1974, when he reacted to his parents' separation by stealing his mother's car. He was sentenced to 18 months in prison. He came out in 1975, his attack on Silvia by then a fading memory; and so, now working with his father as a painter, he had started to spend nights wandering the streets of Plauen, looking for suitable women.

It is not known how many women he attacked at this time, but in 1978 the East German police arrested him for a second time, when he was caught one night strangling a woman in the street. He was jailed for sexual assault, but for only two years and eight months. With parole, he was out the following year.

He told Dr Nedopil that for a while, knowing that the police had his DNA, he tried hard to suppress his inclination. When his parents died within days of each other, he tried to distract himself by caring for his younger sister and brother - "The only worthwhile thing I have ever done," he told the psychiatrist. But soon his siblings were taken to live with his aunt, his restraint began to collapse and, to the best of his recollection, over the next eight years he attacked some 30 different women in the dark streets of Plauen, leaving them unconscious. He tried having girlfriends, but ended up throttling them, too. As far as is known, during this prolonged orgy of assaults, he was not questioned even once.

It seems very likely that, at this time, he got away with at least one murder, making 13 known killings in total. In April 1987 Heike Wunderlich, aged 18, was on her way to college after work when she was strangled and left naked in woods outside Plauen. Twenty years later, police concluded that this must have been the work of Eckert, who was in the area, had no alibi and was to fall into the habit of dumping women's bodies in this way. However, the local police force, which did not question Eckert at the time, continues to insist that it was nothing to do with him. (By curious chance, a second man was also strangling women in Plauen in the mid-1980s, but he had been caught before Heike Wunderlich was murdered. In other words, if this was not Eckert, there must have been three men strangling women in this one small town.)

Later that year, in an incident that should have stopped Eckert in his tracks, he was arrested for a third time, after attacking two young Plauen women who were able to identify him. This time, he was given a serious jail sentence, of 12 years, for attempted murder. The sentence was cut on appeal; he received only a few hours' therapy from a psychologist who heard about his sexual fantasies but concluded he could safely be released; and so, in 1994, after only six years in jail, he walked free.

By now, the Berlin wall had fallen. Eckert moved to Hof in what had been West Germany. In the bureaucratic chaos of post-communist East Germany, paperwork that should have been passed to the Hof police was lost - and Volker Eckert was able to travel, at first across the border to the Czech Republic, with its motorway lined with prostitutes, then, in his lorry, to the far corners of western Europe.

Even after Eckert confessed, in November 2006, to six murders, police attempts to find the truth were hampered by the same pattern of official inadequacy. First, there was a bureaucratic tug of war between the Spanish and the Germans about who should take charge of the inquiry. This was finally resolved in favour of the Germans, and the job was passed to detectives in Hof. But they were already overstretched investigating three murders in the town, and so the job was passed to the organised crime department, the OKD, even though Eckert had no connection with any kind of organised crime group.

The OKD, using up to 40 officers at a time, set out to reconstruct his movements using every conceivable form of record - his credit cards, petrol receipts, his employers' log of his lorry-driving jobs, records from the sleeping policeman in his cab, motorway toll payments and satellite tracking of traffic movements. They sent Eckert's DNA details to 32 different countries and invited them to send them details of unsolved murders of women, so they could be compared with their reconstruction of his movements.

They had some success, uncovering seven more suspected murders to add to the six to which he had confessed, as well as 40 other serious assaults. They finally identified the body of Mariy Veselova, who had lain unidentified in her grave for 26 months. Their Catalan colleagues uncovered a pimping ring in Gerona, run by Russians and east Europeans, as well as unmasking Spanish officials who were taking cash and sexual favours to allow women to enter the country illegally. The OKD visited the families of some of those who had died, travelling, for example, to Russia and to Poland to explain the final chapter of a missing daughter's life.

And yet some of those who worked on the inquiry speak of their deep frustration at the lack of cooperation from many of the European police services to whom they turned for help. Only the Spanish, they say, were enthusiastic supporters, with the Catalan police sending officers to Germany to help in the interrogation of Eckert. There was apparently little or no support from the French, the Belgians, the Dutch, the Czechs - or the British. "We get zero cooperation from the UK," said one source, "unless it is a very serious crime. They did not send us their unsolved murders - except one case, and we looked into it and found Eckert was elsewhere at the time." And there was friction between the OKD and the police in Eckert's old home town, Plauen, who objected strongly to the OKD's claim to have solved the 1987 murder of Heike Wunderlich.

This kind of rivalry and noncooperation played to Eckert's advantage as soon as he drove his lorry across the German border. As one senior official in the inquiry put it: "Our authority stops at the border. He can drive across it but as soon as we try to cross, we need special permission. Everybody claims to be working together, but the justice systems are very different from one country to another. It is a problem now and for the future."

Without adequate working links, the fragmented forces of Europe simply failed to notice the serial killer in their midst. Eckert's other advantage was that the victims themselves were effectively invisible. They had travelled from Africa, Russia and eastern Europe, leaving behind families and friends who might otherwise have reported them missing. Some had entered illegally, some were legal; all lived on the fringe of crime with pimps and other prostitutes who were loth to contact the police when they vanished. And whereas a dead princess may still generate official inquiries more than 10 years after her death, a dead prostitute provokes less interest.

Eckert was not alone in taking advantage of these weaknesses in the system. Delving into European police records, the OKD came across many dozens of unsolved murders of women, particularly prostitutes. They came to the conclusion that there were up to 30 serial killers working undetected across Europe. On the E45 alone - a road running south from Innsbruck into northern Italy - they discovered there were unsolved murders of 45 prostitutes. Even in Eckert's adopted home town, Hof, their colleagues were investigating the deaths of three women - two Thai , one Romanian - which were probably not the work of Eckert. The vulnerability of prostitutes is expressed in the evidence, gathered by the OKD, that Eckert openly told the women he picked up that he would pay them more if they allowed him to tie them up and partially throttle them during sex. Some were desperate enough to accept.

The result of this long history of official failure is not simply that Eckert was allowed to kill for more than 30 years but that even now the full scale of his violence is not known. There are gaps in the narrative, holes in the evidence, and clear indications from his Polaroid photographs and his obsessively detailed notes that the 13 dead and 40 survivors almost certainly fail to tell the whole tale.

Finally, however, it was Eckert himself who prevented the truth coming out. Having confessed to six murders, he then clammed up, apparently upset that the German newspapers were portraying him as a monster. His lawyer, Alexander Schmidtgall, says he felt this deeply: "When he described an attack, he would speak about himself in the third person. He had the emotion rising up in him and he had no way to suppress it. He knew he was an outsider. He was suffering from this." It appears that it was for this reason that he sent the police to fetch medicine from his lorry cab, thus ensuring that he would be forced to stop the attacks.

On July 1 2007, awaiting trial in custody in Hof, he marked his 48th birthday alone, apparently deeply wounded that even his favoured sister, Sabine, considered him a monster and would not visit him. That night, unwatched in his cell, he finally ended his crimes, doing to himself what he had done to so many others: the following morning he was found dead, hanging from the bars of his cell. And without a trial, his story was not told, and so the questions were not asked, and the answers have not been found.