Police officers failed to carry out checks on a psychiatric patient who murdered a woman, which would have revealed that she had killed before, a report has found.
Nicola Edgington, 32, of Greenwich, virtually decapitated Sally Hodkin, 58, six years after killing her own mother.
The police watchdog said officers had failed to carry out a check on the day of the murder in 2011.
She has been jailed for life and told she will serve at least 37 years.
The Old Bailey judge also gave Edgington a minimum sentence of 20 years, which will run concurrently, for the attempted murder of Kerry Clark, 22.
She had tried to stab Ms Clark shortly before attacking Mrs Hodkin, who was a law firm accounts clerk, with a knife stolen from a butcher.
During the trial, the jury heard 999 calls made in the hours before the attacks, during which Edgington said: "I need for the police to come because I've had a nervous breakdown before and I killed someone."
In the hours before the murder, Edgington called emergency services four times asking for help, saying she was hearing voices again and that she was going to kill somebody.
She was taken to two different hospitals, although she was able to walk out.
The Independent Police Complaints Commission said:
- Police in Greenwich were not notified that Edgington was living in the area following her release from hospital in 2009 after she killed her mother
- Officers and police staff did not carry out a Police National Computer (PNC) check during their interactions with her on the day of the murder which would have alerted them to her conviction for manslaughter
- Officers missed an opportunity to use their powers under section 136 of the Mental Health Act when Edgington tried to leave the A&E department shortly after she arrived with police
- Edgington's second 999 call from the A&E department was downgraded because she was considered to be in a place of safety and an officer was not asked to return, despite Edgington saying she could be very dangerous
'Manipulative and dangerous'
IPCC 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 check was carried out which would have immediately alerted them to Edgington's violent history.
"Without this PNC check, both the police and staff at Queen Elizabeth Hospital, Woolwich, were without crucial information which may have influenced their future decisions, increased the urgency of the situation and could have escalated the medical attention she was given."
Ms Green said she hoped both the Met and Oxleas NHS Foundation Trust - which was managing Edgington's care after she was released into the community - would "learn lessons from this tragic case to improve the handling of high-risk individuals such as Nicola Edgington in the future."
The prosecution said Edgington had a borderline personality disorder, was emotionally unstable and regularly blamed others for her situation, while the defence said she had schizophrenia.
John Cooper, QC, mitigating, said Edgington was a woman in crisis and had not been given the help she asked for.
He said she had done what she was told to do; she had called police and taken herself to hospital and had also called her mental health support workers while she was on the bus she took to the scene of the killing, which was also the bus route to Bracton Centre where she had previously been treated.
But Judge Brian Barker told her: "Your actions on leaving the hospital were a consistent and calculated course of criminal conduct.
"You are manipulative and extremely dangerous.
"These were terrible acts and you must take responsibility for what you did.
"The fact you failed to kill Kerry Clark was only due to good fortune and swift reaction. What you did could not have been more selfish."
'My world fell apart'
In a statement read out to court, Mrs Hodkin's husband Paul said there was not a day since the attack that he had not cried.
He said the day he heard she had been killed was when "my world fell apart".
"The thought of not seeing her again has destroyed me," he said. "Over 40 years of marriage were brought to an end by someone who shouldn't have been on our streets."
His solicitor, Daniel Rubinstein, said outside court that the family would be considering further action as questions remained over the authorities' actions.
Marjorie Wallace, chief executive of the mental health charity Sane, said the case was a "shocking indictment" of psychiatric services and all of the agencies involved.
"The most worrying aspect of this is that it could have been prevented. Edgington told people she was not taking her medication and had become a danger to herself and others, yet felt she had to prove it to be taken seriously.
"If the system cannot cope with the relatively few cases of people with mental illness and a history of violence, how can we prevent the stigma which blights the lives of so many thousands who suffer from mental illness and are never violent?"
Edgington killed her mother at her home in Forest Row, East Sussex, in 2005.
On that occasion Edgington pleaded guilty to manslaughter due to diminished responsibility, a plea accepted by the prosecution.
Then she was diagnosed with schizophrenia with emotionally unstable personality traits and was treated as an inpatient in a medium secure psychiatric facility by the Oxleas NHS Foundation Trust.
She was conditionally discharged in September 2009 and moved into a flat in Greenwich where she was being monitored by a consultant psychiatrist, a social supervisor and a community psychiatric nurse.