text: Ryan Matthews, 62, was already serving a life sentence for double murder when he killed Sharon Wall at Wotton Lawn Hospital in Gloucester, where he lived.
  Ms Wall was stabbed twice in the back on 9 July and could not be saved despite efforts from hospital staff.
  Matthews pleaded guilty to the 53-year-old's murder in December.
  Police and NHS investigations have failed to establish how Matthews got hold of the knife he used.
  However Shaun Clee, chief executive of the Gloucestershire 2gether NHS Foundation Trust, said the attack was "neither predictable nor preventable".
  He admitted his staff had "no idea" where he got the murder weapon from but said it was not one of the "sharps" on the unit, which were "controlled" and "moderated".
  Matthews has changed his name several times and was convicted under the name of Stephen Cecil King for two counts of murder and one count of conspiracy to murder in 1983.
  Gloucestershire Police said his mental health had been deteriorating throughout a number of hospital placements since 1999 when he left Broadmoor prison under the provisions of the Mental Health Act.
  He arrived in Wotton Lawn in 2012 but was due to be moved to Ty Catrin Hospital in Cardiff on the day he killed Ms Wall.
  Speaking after the sentencing, Det Ch Insp Steve Porter said: "This is a dreadful case in which a dedicated public servant turned up at her workplace only to lose her life in the most brutal way.
  "People who worked with Sharon say she was a very competent, hard-working healthcare assistant - her family are devastated by their loss.
  "Ryan Matthews had suffered from paranoid schizophrenia and anti-social personality disorder but his condition had been managed with medication."
  Mr Clee said the trust - which is in the process of changing the search policy at the unit - was aware of Matthews' previous convictions, adding "the clinicians were happy with his placement at that time".
  He offered his "deepest sympathies" to Ms Wall's family and said the incident was the most difficult thing he, and many of his colleagues, had dealt with.
  "[Our investigation] has identified some areas where we think we can learn and do additional things, and we are revising and enhancing our search procedures," Mr Clee said.
  He would not confirm specific details about Matthews' care but said it was "common practice for individuals during their treatment programme, at times and when appropriate, to have leave".
  The Health and Safety Executive is investigating Ms Wall's death, saying its inquiry is continuing.
target: A convicted murderer who stabbed to death a worker at a mental health unit has been sentenced to a whole life order.