Judge Nina Lowry said it was not possible to envisage a time when Colin Hatch could be released safely and that 'life imprisonment should mean what it says, namely imprisonment for life'.
The fact that Hatch, 21, from Finchley, north London, had a string of convictions for sex attacks on young boys before he lured Sean Williams to his flat, sexually assaulted him and strangled him, has prompted a review of parole and probation for those convicted of sex crimes.
John Walters, head of the Middlesex area probation service, said Hatch's 'appalling crime' could have been prevented only if he had remained in a secure unit with a long- term programme of treatment.
The two-week hearing at the Old Bailey heard that Hatch had been released on parole 11 weeks before Sean's death last July. The boy had cycled to see a friend in the tower block where Hatch lived when he was abducted and killed. His body was found taped up in bin liners and dumped in a lift.
The jury of six men and six women rejected Hatch's plea of guilty to a lesser charge of manslaughter and his defence that he was driven to attack Sean by a compulsion over which he had no control. The prosecution maintained that Hatch had always planned to murder Sean after sexually assaulting him.
Hatch told police that after he had simulated sex with Sean, the boy was quite scared and had said 'I just want to go home.' He had started walking towards the window 'obviously to shout for help. But I could not let him go. I held him by the throat. He started struggling. I tightened my grip'. Hatch said he got a plastic bag, put it over Sean's head and suffocated him.
John Bevan, for the prosecution, said fantasies 'involving abduction, sexual abuse and the killing of young children' which Hatch had written were found in a wardrobe in his mother's bedroom after he was arrested. Hatch had convictions for attacks on five other boys with his first offence being in 1987 when he was 15.
Just over two years ago, when he was jailed for assaulting an eight-year-old boy, in almost identical circumstances, his own lawyer warned that he could kill when released.
Dr Anthony Wilkins, a psychiatrist, also urged the court at the previous hearing that Hatch should be sent to Broadmoor top security hospital. But doctors at Broadmoor had not considered Hatch dangerous enough and he was jailed for three-and-a-half years.
Judge Lowry told Hatch: 'Three years ago when you were before a court for offences committed against young boys, the view of the medical authorities differed and consequently the court had no power to order you to be detained indefinitely.
'It is plain to me that when at liberty you are highly dangerous to the public. Medical opinion is you are likely to remain so for the foreseeable future. That is the reason why I have decided a minimum recommendation (on length of sentence) is not appropriate. It is not possible today to envisage when you could be safely released from prison and as of today life imprisonment should mean what it says - namely imprisonment for life.'
The verdict was greeted with cheers in the public gallery, where Sean's mother and father, Lynn and John Williams, were sitting. Mrs Williams said later that it could not have been a better verdict and meant Hatch would 'rot' in prison. 'But it doesn't help that he is still living, still breathing - Sean's not.' The family said others would have to take some of the blame for Sean's death and questioned why he was not more closely supervised by probation officers and why doctors had not discovered his true condition during more than 40 visits while he was at a young offenders' institution.
Mr Walters said Hatch had been identified as being a potential danger to the public. But he added: 'Observation of his behaviour following his release and what he led his probation officer to believe did not appear to justify a recommendation that he be recalled to prison.'
Mr Walters was satisfied the staff responsible for Hatch provided a proper level of supervision except over the lapse of time in referring him to a psychiatric clinic which was part of his parole conditions.
One appointment with a psychotherapist at the Portman Clinic, Hampstead, who specialised in sex offenders, was cancelled by Hatch and a second by the consultant.
A review of procedures following the case had led to the introduction of improved risk assessment and a comprehensive training programme for probation officers responsible for supervising sex offenders, Mr Walters said.
The service also now required that where a parole licence included a psychiatric requirement an appointment was made before release for the parolee to be seen by a psychiatrist within 14 days of leaving prison.
Det Supt Duncan Macrae said after the case that if the authorities had been in possession of all Hatch's writings, the murder may not have happened. 'He was declaring he would tell the authorities what they wanted to hear in order that he could get parole and be released.'
Det Supt Macrae added: 'He made it difficult for them to get inside his mind while he was planning for his release and what he was going to do to the young and vulnerable in our society.'
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