Sharon Carr, a girl obsessed with death and violence, secured her place in criminal history yesterday as Britain's youngest female murderer.

She had killed at the age of 12 - a savage attack in which a teenage hairdresser was mutilated with 29 stab wounds. The victim, Katie Rackliff, had been picked out at random as she walked home from a nightclub in June l992.

The trial at Winchester Crown Court was told that in the years that followed, Carr seemed to be exultant over the killing, and yet haunted by it. She was endlessly writing about the murder and drawing pictures of a knife.

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Samples of her notes were graphic. In one she said: " I am a killer. Killing is my business - and business is good." In another: " I was born to be a murderer. Killing for me is a mass turn-on and it just makes me so high I never want to come down. Every night I see the Devil in my dreams - sometimes even in my mirror, but I realise it was just me."

Four years after the murder, a diary entry stated: " I bring the knife into her chest. Her eyes are closing. She is pleading with me so I bring the knife to her again and again. I don't want to hurt her but I need to do violence to her ... I need to overcome her beauty, her serenity, her security. There I see her face when she died. I know she feels her life being slowly drawn from her and I hear her gasp. I guess she was trying to breathe.

"The air stops in the back of her throat. I know all her life her breathing has worked, but it does not now. And I am joyful".

Were these the fantasies of a deeply disturbed mind, as the defence claimed? Or, as the Crown held, the grim memories of an " evil and precocious" schoolgirl who gloried in what she had done? The jury had no doubt.

It was the writings, and subsequent verbal confessions that convicted Carr. There was no forensic evidence, but, as the prosecution pointed out, she had knowledge of the murder not available to the public. She graphically described one particular injury, details of which the police had deliberately withheld, and she also knew that a bracelet had been stolen from Katie Rackliff - knowledge that only the killer would have.

In June 1994, almost two years to the day after Katie's murder, Carr attacked a pupil called Ann-Marie Clifford with a knife, for no apparent reason, at Collingwood College Comprehensive in Camberley, Surrey.

While awaiting trial, she was sent to an assessment centre where she tried to strangle two members of staff. Two counts of actual bodily harm were taken into account when she was convicted of wounding Ann-Marie, and sentenced to be detained at Her Majesty's pleasure.

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At Bulwood Hall young offenders' institution, staff alerted police after Carr began talking about the killing of Katie Rackliff on the telephone to her friends and family and wrote about it in her diaries. She also began to give details of what she had done to a prison officer, on whom she had developed a crush, as well as talking about it to a probation officer.

The Rackliff killing had remained unsolved despite four years of intensive investigation by police. Some of the knife blows that Katie suffered in the attack had gone straight through her body and out the other side. Her sexual organs were mutilated, and her clothes pulled up, although there was no evidence of sexual assault.

Detectives seized Carr's writings and drawings, and questioned her for 27 hours. She gave three different accounts of how Katie had been killed, but in all of them the central theme was she had repeatedly stabbed her.

In two of the versions, Carr said she was with two boys in a car at the time of the attack, and they had engaged in sexual activity with Katie before dumping the body. She named the two boys. Police interviewed them but they provided alibis for each other, and were eliminated from the inquiry. However, the prosecution could not satisfactorily explain how Katie, who weighed 8st 8lbs, was dragged across a pavement and around a corner by a 12-year-old girl.

Carr continued with her writing even after being interviewed by the police. In April l996, the month before she was charged, she wrote: " I am not like one of those pretty girls who breaks down due to a guilty conscience. Through six and a half years of causing people grief, I still have not found one." On 7 June, her diary read: " Respect to Katie Rackliff. Four years today."

Sadistic violence seemed to be part of her life. Police discovered that she had decapitated a neighbour's dog with a spade, and there was also a "suggestion" from a friend that she had fried live hamsters.

Detective Sergeant Paul Clements, who interviewed Carr extensively, recalled: "It was almost as if she was in another world. What sticks in my mind about talking to her was the coldness. Most people that you interview show some feeling as to why they have done what they have done. But with her there was a complete absence of emotion and reason."

Carr was born in Belize in l981 and was brought up by her mother and stepfather - a soldier. After moving to England the family settled in Camberley, Surrey. Her parents split up and she was briefly fostered, but after a month she returned to the home of her mother. At school, her teachers initially described her as polite and helpful, but her behaviour deteriorated and she became disruptive and attention-seeking.

Criminal psychologist Gordon Tressler said: "This is a difficult case to understand. One can find precedents of young children killing other young children, but in this case it was a child killing someone who was almost an adult.

"This is an extremely dangerous person because she is clearly prepared to kill without an adequate motive. That makes her conduct very unpredictable and very dangerous. She is a great danger to the public."