Howard, now 61, is serving a life sentence in Frankland Prison, Durham, for raping and murdering 14-year-old Hannah Williams in Kent in 2001. He is the only suspect for the murder of 15-year-old Arlene Arkinson in 1994, and in recent months he has been questioned about the rapes and murders of several other women and girls in Ireland and England. He is known to have attempted to rape a six-year-old child in 1965, when he was 21, a young woman in 1969, and an older woman in 1973.

Gahan, now 28, is angry. Howard got away with what he did to her. Hannah Williams' mother is angry. She believes Howard should have been in jail at the time he preyed on her daughter. The Arkinson family is angry, believing he could and should have been convicted of murdering Arlene, whose body has never been found. "We need a full public inquiry into how this man got let go again and again to do the things he did," says Arlene's sister, Kathleen.

The case of Robert Howard suggests that the authorities - from the police to the prosecutors to the judiciary - simply haven't taken rape seriously enough. Howard got nine days in borstal for the assault on the six-year-old. He got six years for attempted rape of the young woman, and was free within three. After he raped the older woman, a psychiatrist warned that Howard was a dangerous psychopath and should be jailed for as long as possible. He got 10 years and was out in seven.

In 1994, after Howard raped Gahan, another psychiatrist warned that he was dangerous to women and becoming more so. He got a suspended sentence. Howard moved easily between the Republic of Ireland and the north, and across the Irish Sea to England and Scotland. He was rarely monitored, and never for long. Liaison between police forces was minimal.

Gahan, who has moved back to the Irish midlands where she was born, is still deeply traumatised by the violence Howard inflicted on her. She left home as a 15-year-old emerging from a troubled childhood. Her mother had been killed in an accident when she was five. "My father was finding it hard to control us," she says. "There were 10 of us. My friend had moved up to the north and I decided to follow. I was running away from Daddy, really, but it wasn't his fault. I was wild."

Her friend's boyfriend knew a middle-aged woman called Pat Quinn, who said Gahan could stay in her house in Castlederg, where she lived with her own teenage daughter, Donna. Castlederg is one of those small Northern Irish towns that is well described by Yeats's phrase, "Great hatred, little room." It was deeply afflicted by the Troubles.

Gahan liked it. She got a job washing dishes in a Chinese restaurant. She became part of a circle of teenagers, including Donna and Arlene Arkinson. Their social life revolved around nights in bars and discos in Castlederg and neighbouring towns in Tyrone, and in Donegal in the Republic, reached by a network of narrow, mountainous roads through woods and bogs. The border meanders crazily across this lonely territory. Robert Howard, then nearly 50, was Pat Quinn's boyfriend, though he was more interested in the teenage girls he met through Donna. He let them stay at his flat on Main Street.

"He knew I had nothing," says Gahan. "He knew everything about me. He bought me cigarettes and runners and things. He used to bring us up to the bog to cut turf. He brought me out for drinks. I knew him as Bob. He called me Mick. He was so nice to me. He was from the south, like me. I'd left Daddy, and he was like a daddy who let me do what I wanted. I thought the world of him."

A few months after Gahan had arrived in Castlederg, Howard suggested an elaborate plan to her. Ostensibly, he was fixing her up with a young taxi driver she fancied. She was to tell everyone she was going away for the weekend, get a bus to the next town, and then come back secretly to meet Howard - and the taxi driver - at his flat. Except the taxi driver never came.

Howard took Gahan to a pub in the village of Sion Mills that night, but left when they saw someone they knew. "I was afraid," says Gahan, "but I don't remember why. When we got to the flat, I had a pounding headache. He gave me some pills. I remember sitting on his knee. I remember nothing then until I woke up the next morning naked in his bed. He started coming on to me, and I said no. He said I hadn't said that last night. He started getting annoyed then, and that is when he put the rope round my neck."

When she escaped, after three days held captive, Gahan told police she'd been raped, but she felt they didn't believe her. "They banged the table and shouted at me," she says. They wanted to know why she hadn't tried to get away sooner, and why she did not, initially, tell them about her interest in the taxi driver. Looking back, she can see the control Howard had over her. "It was the way he had me thinking," she says. There was compelling evidence to back her account of what had happened: strangulation marks on her neck; her fingerprints on the windowsill from which she had jumped; a rope.

Gahan was taken to a children's home and soon afterwards returned to her family, then to a job in England. Howard was arrested and released on bail. He was ordered to stay with the Quinns, even though this was a household that included the teenage Donna. He was instructed not to go out at night, or into pubs. In the summer of 1994, Gahan was informed by the police in Northern Ireland that her case was coming up. She returned to Ireland, but was not, in fact, called to give evidence. Howard had originally been charged with five rapes and with buggery. But the charges were dropped.

Instead, Howard agreed to plead guilty to unlawful carnal knowledge. The implication was that Gahan had been a willing partner and that the offence lay only in the fact that she was, at 16, under the legal age of consent (17 in Northern Ireland). Her statements had been heavily edited. A prosecuting lawyer told the court that no rope had been found. Judge David Smyth ordered that a psychiatric report be prepared on Howard, and said that if it was satisfactory, he would not be sent to jail. He was released again on bail. This was extraordinary - by that stage, Howard was suspected of having murdered 15-year-old Arlene Arkinson.

On August 13 1994, Arlene was babysitting at her sister Kathleen's home on a housing estate in Castlederg. Kathleen returned home at about 11pm, and soon afterwards Donna Quinn arrived to pick up Arlene. "She said it was her and her boyfriend and her mother and Bob Howard that were going out," says Kathleen. They were going to a disco at the Palace Hotel in Bundoran. An old-fashioned resort full of boarding houses and bars and amusement arcades, Bundoran is perched on the chilly edge of the Atlantic in Donegal. Donna said they'd be back by two the following morning. Kathleen said good night to her youngest sister. She never saw her again.

"Arlene was wild, like me," Gahan recalls. "We were alike, too, because I had no one telling me what to do and nor had she. We got on great."

Arlene's mother had died when she was 11. She lived between the homes of her older brothers and sisters, though she sometimes returned to her father's house. "I used to hear her upstairs, crying, 'I want my mummy,' " he remembers, as he sits in his living room, looking at photographs of his lovely, laughing daughter, missing now for more than a decade.

At first, after Arlene disappeared, the Quinns covered up for Howard, claiming Arlene hadn't gone to Bundoran. However, it quickly emerged that after the night out in Bundoran, Howard had dropped off the others before driving away with Arlene. He claimed that he had dropped her off in Castlederg, and that he'd seen her in a car in the town the next day. He was not believed. Pat Quinn admitted Howard had got back home much later than she had originally said. The terms of his bail on the charges concerning Gahan included a curfew, which he'd broken, but still he was not held in custody.

A petrol bomb was thrown into the Quinns' house. Howard was driven out of Castlederg by members of Arlene's family. He sold the car he'd used the night Arlene disappeared. For a time, he lived rough in a van across the border. Again, he was moved on by local men who knew his reputation. It was a full six weeks after Arlene disappeared before he was arrested. One of Arlene's sisters recalls something a local RUC officer said at this time: "He said, 'I wish I could show you that man's file. You wouldn't believe it.' " Still, at the time he was neither charged nor brought to trial.

As an adult, Robert Howard called himself the Wolfman and the Wolfhill Werewolf. He gave himself a new middle name, Lesarian, believed to be a reference to a mythical child killer. He was born in Wolfhill, County Laois, in the south-east of Ireland, in 1944. He was a tall, gangly boy, bright enough at school, according to some of those who knew him then, but edgy, easily distracted. "He mitched [played truant] a lot," says one contemporary. "His father worked in the local brick factory and drank heavily in the local pub. His mother had nine children to raise. There would have been very little money brought home."

By the time he was 13, Robert was in trouble. Convicted of burglary, he was sent to St Joseph's Industrial School, in Clonmel, not far from his home area, a reformatory run by priests and brothers. The truth about how such Irish Catholic institutions were run has begun to emerge in recent years. "These people were monsters," says one former inmate. "The place was unbelievable. We were starved. We were beaten with leather straps with coins sewn into them until we were bleeding. We had to gather turnips and stones for the local farmers. You had no name. You had a number. There were boys in there going around like zombies. We were terrified, all the time - a lot of us were damaged for life. Love was never spoken of, never shown. There was never a comforting word. Just relentless violence." There was also sexual abuse, and Howard would later claim to have been a victim of this.

On his release, his father threw him out of the family home. The 16-year-old lived rough in barns and outhouses, and possibly in old shafts and tunnels from the days when Wolfhill had coalmines. One man, a child in the 1950s, recalls making a disturbing discovery in his father's hayshed. "We found old cans and blankets and things - Howard must have been sleeping there. We also found a diary. It was all about how he wanted to break into women's houses when they were in the bath, and the violent things he'd do to them. My father caught us reading it and took it away."

Another local man recalls being told by a neighbour that, while out hunting one day, he had come upon a local farmer performing a sexual act on the teenage Howard in the woods. The man didn't intervene, but fired a shot into the air.

Howard continued to steal. He'd rob from local shops and take cars. He was sent to a second reform school, St Conleth's, in Daingean, County Offaly, another of the most notorious of Ireland's brutal institutions for young offenders. A former priest who worked there said the priests were "programmed with an extraordinary level of violence" and that "most of the boys ended up totally disturbed".

Howard moved to England. In 1965, when he was 21, he broke into a house in London and ordered a six-year-old girl to undress, claiming he was a doctor. He attempted to rape the child, and hurt her. A week later he returned and tried to break in again. This time he was caught. His punishment was nine days in a borstal, after which he was sent back to Ireland. At that time it was common for Irish criminals to be sent home in this way, a system of informal deportation. He didn't stay.

In 1969, Howard broke into the home of a young married woman in Durham and attempted to rape her. She ran, naked and screaming, from the house. He grabbed her by the throat before neighbours managed to drag him away. He was sentenced to six years in jail, and served three, in Frankland Prison. During his time there, he assaulted a female member of staff. By 1973, he was free, and was again sent back to Ireland. Using the name Lesley Cahill, he got a factory job in the then prospering seaside town of Youghal, County Cork.

One night in May that year, a 58-year-old woman who lived alone woke up to find Howard in her bedroom. He had broken into her house, which was beside his lodgings. He made her hand over her money and keys, then forced her upstairs again, breaking her ankle in the process. He tied her to her bed, stuffed her mouth with cotton wool and raped her, before driving off in her car. "She was a very vulnerable person," says Willie Doyle, then the local Garda sergeant. "She might have suffocated, but luckily for her some relations called the next morning and found her. She was very traumatised."

Howard was arrested at Dublin airport. "He was very soft-spoken," recalls Doyle. "You would never imagine he could be violent." Psychiatrist Dr David Dunne, who interviewed Howard at the time, says he, too, was surprised by Howard's "extremely courteous" demeanour. "He was a very refined man, but I had seen his record and knew he was also extremely dangerous. I sensed and feared he had already killed someone. I knew his violence was likely to get far worse, especially towards women. I believed him to be an explosive psychopath. I wanted him to be sent to jail for an indefinite period."

Howard could have got life. Instead, he got 10 years. He was out again in 1981. An internal Garda bulletin noted that he had returned to Wolfhill and "local opinion is that he is not going to waste any time before returning to his old tricks".

A woman who remembers him from this time had her own horrific experience of sexual brutality. She was the teenager who would become known a decade later as "the Kilkenny incest victim". In 1981, aged 15, she was pregnant with her father's child. He was beating and raping her routinely, and used to take pornographic pictures of her which he'd show to other men in local pubs.

"Bob used to come to our house sometimes at night, and he and my father would drink whiskey and poitín together," she says. "My father would say to him, 'Where have you been?' He'd say, 'I've been visiting relations.' My father would laugh. I always felt it was some sort of code. He was creepy. They were birds of a feather." Her father was eventually jailed for seven years.

In 1983, Howard got married to a young woman he met in a Dublin hospital. The marriage lasted three years. They lived at various addresses in Dublin. She, too, was described as vulnerable, with emotional troubles; her friends revealed recently that Howard was violent and cruel to her. In 1988, he was jailed for 15 months for larceny. He went to Northern Ireland in 1990 to attend an alcohol treatment unit run by nuns in Newry, County Down. It was at around this time that he met Pat Quinn and moved to Castlederg.

He lived at first in a caravan park on the edge of the town, a down-at-heel place where many of those awaiting public housing lived. Not long after his arrival there, in 1991, a young woman came from Dublin to stay with him. He was 47, she was 22. He tied her up and raped her repeatedly, was extremely violent and kept her as a prisoner until, three weeks later, her parents arrived and took her home.

The woman became pregnant as a result of the rape and now has a 14-year-old child. The details of what happened to her did not emerge until her family told gardaí six years later, in 1997. Police decided she was too vulnerable to give evidence and Howard was not charged. His next known victim was Gahan.

"He has a strong desire to dominate teenage girls both sexually and physically," wrote Dr Ian Bownes, the forensic psychiatrist asked in September 1994 to provide an assessment of Howard to assist Judge Smyth in sentencing him for the "unlawful carnal knowledge" of Gahan. "He has the propensity not only to commit further offences of a similar nature ... but also to escalate his offending behaviour." His activities were premeditated, involving the identification and targeting of "a vulnerable victim" and the use of a "sophisticated grooming process". Bownes said he was pessimistic about Howard's ability to undergo any treatment programme - his pattern of behaviour had been established over many years and would be "extremely resistant to change".

When Howard appeared in court again for sentencing in January 1995, despite the damning psychiatric report, Judge Smyth gave him a three-year suspended sentence. As he freed Howard, the judge told him to stay away from teenage girls. Bownes heard the news on the radio. "I was somewhat surprised by the leniency of the sentence," he says. "In retrospect, we can see the system failed disastrously." Bownes never saw Dr Dunne's 1973 report on Howard. He was not told that Gahan had alleged Howard used a noose.

What Howard would later refer to as "dark days in Castlederg" were over. In March 1995, he moved to Scotland, where he told local housing authorities that he had left Northern Ireland "in a hurry". He said the IRA was after him.

He was given a flat in the rough Glasgow suburb of Drumchapel, near two schools. Pat Quinn came over from Castlederg to join him. The Northern Ireland police informed Strathclyde police about Howard's criminal record - and that he was the chief suspect for the murder of Arlene Arkinson.

Howard returned to Ireland several times, but kept his Scottish base. Pat Quinn left in October, by which time Howard already had another girlfriend, a woman he'd met in a pub. She had a 10-year-old daughter. Then the Sunday Mail, presumably acting on information obtained from the police, outed Howard. The newspaper printed a photograph of the "Face Of Evil" over a piece about the "twisted child sex fiend" that detailed his criminal record and described him as "one of Ireland's most dangerous sex offenders". The sub-headline had a simple message: "Kick him out!"

Within hours, a crowd had descended on the tenement house and the windows of Howard's second-floor flat were smashed. He used a rope to escape from a back window.

Howard was on the move again. A police surveillance team located him at a hostel in Hither Green, south-east London, but he was hounded out by other residents who found out about his past. He disappeared for a time, and was later traced to Brockley, then to Catford. A child protection officer noted at the time that Howard was a difficult subject to monitor.

By 1999, he was living with a woman called Mary Scollom at her house in Northfleet, Kent. Scollom had formerly been involved with the father of Hannah Williams and had remained friendly with the girl after the relationship ended. Scollom would take her for walks with her dogs around the Blue Lake at the back of her house.

Hannah Williams' parents had separated before she was born. When she was four, she had been sexually abused by a boyfriend of her mother's and had spent some time in care. In 2001, she was 14 and living with her mother in the outer London suburb of Deptford. She had learning difficulties and was described as immature.

Howard met Hannah through Scollom and showed a great interest in her. In February 2001, he took a home video of her, cuddling the dogs at the house he and Scollom shared. On April 21, she left home to go shopping in Deptford market, around the corner from her home in Elgar Close. She had very little money, but she liked looking at clothes. Her brother, Kevin, who had a Saturday job in the market, heard her answering her mobile and having a very brief conversation. She told the caller, "I'm going now."

By 10pm that night, Hannah's mother, Bernadette Williams, was worried. Hannah had been supposed to meet a friend that evening but hadn't shown up. She wasn't answering her mobile. By 5am, Bernadette was frantic. She went to the police. She felt she wasn't taken seriously - several officers have since been disciplined for their role in the initial stages of the investigation. Bernadette made her own "Missing" posters and took them round local shops and bars. But Hannah was gone.

Almost a year later, a workman was using a digger to clear dense undergrowth on land near the Blue Lake at Northfleet as part of the Channel Tunnel development. He found a badly decomposed body wrapped in a blue tarpaulin. Police initially thought it might be another missing girl, but when they released a description of the clothes, Bernadette knew it was her only daughter. "I finally found out my daughter was dead, and that her body had been found, by watching it on the telly," she says. "To find out that way was unforgivable. I screamed and then I cried and cried."

Hannah's coffin was placed in a carriage drawn by white horses. "She would have made a beautiful bride," says her mother. "But instead of a white wedding, we had a white funeral."

Hannah had been raped and strangled. The blue rope that had been used to kill her was still wrapped around her neck. Howard was arrested in March 2002 and charged with her murder. During his trial, at Maidstone, Kent, in October 2003, it was revealed that he had used his girlfriend's mobile to call Hannah to her death.

Detective Inspector Colin Murray (now retired), who led the investigation, had no witnesses and no DNA evidence. However, he had circumstantial evidence and he was also able to rely on "similar fact" evidence. A traumatised young woman gave evidence that Howard had brought her to the same place where Hannah Williams' body was later found, and that he had tried to sexually assault her. She had escaped.

Gahan came over from Ireland to give evidence that showed Howard's grooming techniques. Crucially, she also described how, when he was raping her, Howard had put a noose around her neck. Kathleen Arkinson gave evidence about Howard's part in Arlene's disappearance. It took the jury just three hours to convict him. Sentencing him to life imprisonment, Mr Justice McKinnon said, "It is clear that you are a danger to teenage girls and other women, and have been for a long time."

Reporting on the trial was severely restricted because Howard had, by this time, also been charged with murdering Arlene. "Mrs Williams hugged us at the end of Hannah's trial and said, 'See you in Ireland,'" says Kathleen Arkinson. "We assumed that she would be called, and the others, too." But the public prosecution service in Northern Ireland did not attempt to introduce similar fact evidence. It has not explained this decision.

The jury that heard the case in Belfast's crown court in the summer of 2005 knew nothing of the patterns of behaviour Howard had established in a career of sexual violence that spanned four decades. He was acquitted. Weeks later, he was also acquitted of other charges of sexual abuse against a 17-year-old girl in the 1990s.

Police from Northern Ireland, the Republic and England have already held a one-day conference to discuss other crimes with which Howard might be connected. The police ombudsman for Northern Ireland, Nuala O'Loan, has launched an inquiry into the handling of complaints against Howard there. Gardaí in the Republic have applied for permission to question Howard in connection with the disappearances of at least two young women in the 1980s and 1990s. British police questioned him earlier this year about 13-year-old Amanda "Milly" Dowler, abducted and murdered in 2002.

Barry Cummins, a journalist who has written a book about missing Irish women and children, says a thorough investigation into Howard's life is now needed. "This was a man who travelled freely all over Ireland and the UK, and lived in many places," he says. "The police should be looking at all unsolved disappearances, murders and sex crimes against women and girls during the periods when he was at large. They should be asking, 'Where was Howard?' "

Ireland established a sex offenders' register only in 2001, and liaison on the monitoring of sex offenders between the authorities in the North and the South, and between Ireland and the UK, is seriously underdeveloped.

Howard was a skilled hunter. He carried out random attacks on some of his victims, while others were groomed. He could live rough, but also knew how to play the system to get accommodation. He favoured poor areas. It was easy to win women with low expectations. He tracked down socially marginalised women to use as cover while providing access to vulnerable young girls. The ones he chose had typically already had bad experiences with men, and were relatively unprotected. He faked kindness, and deceived many.

In the desolate boglands around Castlederg, the search for Arlene Arkinson's body continues.