A campaigner has called for an urgent review into how a killer was able to murder his ex-girlfriend despite having killed two other former partners.
Theodore Johnson admitted murdering Angela Best after she met another man.
Julian Hendy, of Hundred Families, said there were "serious questions" after it was revealed the 64 year old had two previous manslaughter convictions.
Camden and Islington NHS Trust, which provided care to Johnson, said it would share its report with Ms Best's family.
On Tuesday, Johnson pleaded guilty to beating Ms Best, 51, with a claw hammer and throttling her with a dressing gown cord at his Islington home on 15 December 2016.
The pair had met in the mid-1990s shortly after he was released from a secure mental health unit following the deaths of two other women.
In 1981, Johnson was jailed for three years for the manslaughter of his wife Yvonne Johnson, while in 1993 he pleaded guilty to manslaughter on the grounds of diminished responsibility for strangling former partner Yvonne Bennett.
He was sent to a secure hospital indefinitely but released after about 18 months.
Mr Hendy said the fact that Johnson had pleaded guilty meant the background to the case and the reasons why he was released "will not be examined fully and openly in court".
"The public will want to know what the services that were supposed to be monitoring him were doing," he said, and called for a "robust and rapid investigation".
Camden and Islington NHS Trust, which began providing care to the 64-year-old in December 2004, said Johnson had been "seen in compliance with the conditions imposed by the Mental Health Tribunal who oversaw his earlier discharge".
"Following the conclusion of legal proceedings, we will be sharing with Ms Best's family the Trust's own independently-chaired report into the care provided," the trust said.
Johnson will be sentenced at the Old Bailey on Friday.