Scotland Yard has rejected criticism of its treatment of convicted killer Nicola Edgington, who told officers she was on the verge of committing another murder hours before stabbing grandmother Sally Hodkin to death.
Hodkin, 59, was almost decapitated by mental health patient Edgington, 32, during the attack in Bexleyheath, south London in 2011.
She launched the assault with a butcher's knife after being fended off by another woman she attacked, Kelly Clark.
Edgington called 999 five times in the hours before killing Hodkin, telling operators she was hearing voices and would kill again.
Edgington murdered her own mother in 2005. Police took her to Queen Elizabeth Hospital but she was allowed to leave within four years.
A probe by the Independent Police Complaints Commission (IPPC) spoke of "great concerns" at the failings, specifically a fiailure to carry out a basic background check. IPPC commissioner Sarah Green said: "While our investigation found that no police officers or staff breached the code of conduct, it is of great concern that no PNC [police national computer] check was carried out which would have immediately alerted them to Edgington's violent history.
"We also found that when Edgington tried to leave the hospital shortly after she arrived, there was a missed opportunity for the officers to use their powers under section 136 of the Mental Health Act which would have provided medical staff with the opportunity to detain her."
In response to this criticism, Scotland Yard Commander Neil Basu said: "A PNC check alone would not have prevented the actions that Edgington subsequently took.
"There has been criticism that officers did not conduct a Police National Computer (PNC) check on her details, which would have identified that Edgington had been violent in the past and had a previous conviction for manslaughter.
"With the benefit of hindsight this may have assisted officers with background information, however, as the IPCC investigation concluded, whilst Edgington was vulnerable, she was compliant and herself seeking medical assistance.
"The officers acted on the facts in front of them, which they do on a regular basis every day."