Zahir Aziz was assessed as 'low risk' when he came out of prison for GBH. But he went on to murder his uncle, Shabir Choudary, from Wakefield, in January 2010 - just a month after his release.
West Yorkshire Probation Service have now admitted he should have been assessed as 'high risk', which could have led to him being recalled to prison when he breached his terms.
Mark Siddall, Director of Operations for West Yorkshire Probation Trust, said: “On behalf of West Yorkshire Probation Trust I would like to express our sympathy with the family of Shabir Choudary for their loss.
"It is extremely rare for an offender under our supervision to commit a serious crime, but every case is one too many.
“A thorough investigation of this case was carried out and it found that, while the overall case management was good, the initial risk assessment carried out fell short of the appropriate professional standard."
Nesar Rafiq, Choudary's nephew, is also campaigning for Serious Offence Reviews to be available for the family if they want to see them. He is being supported in this by Wakefield MP Mary Creagh.
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