It's every mother's nightmare: your teenage daughter goes missing. For this mother it was worse - she was blamed. By Decca Aitkenhead
Early on 30 September, Mercy Zani-Merriman was disturbed by a noise at the door of her small terraced house in Bradford. She thought it was her 15-year-old daughter, Nobantu, coming home after two weeks staying with friends. Instead it was a policewoman who said she was checking for fingerprints: "You reported a burglary, didn't you?" Mercy hadn't; the officer left.

But the officer had not been checking for fingerprints. She had been trying a key in the door - a key from the pocket of a body found up on the moors a day earlier. It fitted. Within the hour, officers were back at the house to tell Mercy that her only daughter had been murdered, strangled with her own scarf.

It is every mother's nightmare. A headstrong, independent teenage girl unwilling to accept parental discipline, disdainful of school, who falls into questionable company. For most parents the trauma is short-lived and not that painful: the errant daughter does not stray far and after some attacks on authority returns to the fold. But Mercy and Nobantu took the drama of teenage daughter and despairing mother to extremes.

In the weeks after her daughter's death, Mercy - and most of the city - was told that the 15-year-old had been a determined truant; a regular in the cafes and streets of Bradford's red-light district; and, quite probably, a child prostitute. Headlines leered over the dead girl's "secret double life". Mercy became the model of an awful, neglectful mother. "What kind of woman could let a daughter loose like that?" the locals asked. She was abused in the street and shunned by her neighbours.

At times it seemed as though the entire city had turned upon Mercy, venting its moral outrage on her. She was culpable; she is frank about her failings as a mother. But what she points out - and what the community has been less prepared to accept - is its shared responsibility for what happened.

As usual, the story is more complex and troubling than it first appeared. Nobantu did lead a double life, but there was nothing secret about it. The social services, her school and the educational welfare office were all well acquainted with her parallel life. They did nothing about it. Her mother's plea to have her taken into care was ignored. Despite the weighty provisions of the 1989 Children Act, a child dangerously and publicly out of control was allowed to pursue a path that ended on the Yorkshire moors. The state was well armed to deal with vulnerable children, but when it came to the test, it failed to act.

Mercy Zani-Merriman arrived in England in 1987 as a political refugee, the widow of a Pan-African Congress member assassinated in South Africa. In 1990 her children - two sons, now 19 and 10, and Nobantu - who had been staying with their grandmother, joined her in Bradford. They attended the local schools while their mother studied to become a social worker.

Things started to go wrong last February: Nobantu began to play truant. "The school welfare officer came to the house and told me Nobantu was pregnant. I knew there were rumours going round her class - that's why she stopped going to school. But she wasn't pregnant." Mercy is a slight, softly spoken woman in her forties - quick to give a wry smile, quick to fall silent. "I begged her to go back to school."

Instead, she went missing - the first time, to Bradford's red-light district, from where the police duly brought her home. She began staying away for weekends, supposedly at a friend's house; when Mercy tried to lock her in at home, she escaped through a window and spent a spell in a notoriously seedy Bradford guest house. By April she was off again, this time to London. She returned within weeks, but had by then abandoned school altogether.

"She was beyond my control. I couldn't physically drag her to school. I pleaded with her, I told her about a black girl from Leeds who was found dead two years ago. I used to say to her, 'Nobantu, you are going to end up like that girl', but she just thought I was a spoilsport.

"The school welfare officer used to escort her to school, but she would disappear. So he stopped coming. As a last resort, I wrote to social services and begged them to take her into care. They said it was not appropriate in the present economic climate."

Under the Children Act, a child suffering or likely to suffer significant harm must be investigated by the local authority. Where "harm" - which includes impairment of physical health and social, intellectual or behavioural development - is attributable to the child being beyond parental control, the child may be placed into care. Mercy was, and is, frank about her failure to control her daughter. "What parent wants their child taken into care? Normally parents are fighting it. Here I was, begging them." But the social services - which had worked extensively with the family - merely referred her to the local Barnardos office.

Nobantu stayed at home over the summer but when the school year began in September she refused to attend. A schoolfriend, a "respectable girl" who lived near the school, suggested Nobantu come and stay for a few weeks, and assured Mercy she would see she attended. For a week or so Nobantu would pop back to change clothes. Mercy assumed she was going to school.

"But then she never came back. I kept thinking it was because she had spent the last pounds 2 I had given her on cigarettes, and not a bus pass."

When the news broke that a 15-year-old girl had been found on the moors, dead for some time, it seemed inconceivable that nobody had reported her missing. Why had no one noticed?

The school headmaster, John Hull, was quick to declare his conscience clear. Educational welfare officers had made repeated contact with Nobantu's mother when she first began truanting in the spring. But Mr Hull acknowledges that although Nobantu's school attendance totalled less than a week between February and her death in September, she was not referred as a priority.

"It is a simple question of resources. By the time you are 14 or 15, education is basically not compulsory in this country any more," he says. When Ofsted inspectors visited the large comprehensive over the summer, they advised the school to prioritise its educational welfare work on pupils who could be brought back on to the straight and narrow. In effect, that meant writing off confirmed and unreformable truants.

"By September, Nobantu's truancy had passed the point where our intervention could be expected to make a difference. There were other, more important, cases than Nobantu Zani. And there weren't enough funds."

Mr Hull also attacks a change in the law that allows parents to move children from school to school, for creating "turbulence" - a confusion that can muddy attendance records. A suggestion in June that Nobantu would move school had helped to deflect attention from her absenteeism. For his part, the director of social services, Liam Hughes, says that as Nobantu would not have willingly entered its "Looked After" children-in-care system, a care order would have been pointless; she would have simply disregarded it.

Mercy's distraught refusal to give interviews when the story broke helped to provoke wild reports that she had not seen her daughter for 10 weeks, and neither knew nor cared that she was probably a prostitute. Mercy Zani-Merriman now knows her daughter had not been staying with her friend or going to school. She had been seeing a man alleged to run brothels in the city. Mercy believes her killing was wrapped up with prostitution. But longstanding tensions between ethnic communities and the police have silenced people with information to offer. The likelihood of the killer being caught fades daily. Meanwhile, with the killer still at large, many people in Bradford have decided they know enough to decide who the real culprit is.