A violent offender killed his partner three days after being released from police custody over an earlier alleged assault on the woman in Northern Ireland, a watchdog said.
Six officers were disciplined for failures surrounding the murder of Caron Smyth by Shaun (Sean) Hegarty in Belfast in December 2013.
Hegarty was sentenced to at least 18 years in prison. Co-accused Ciaran Nugent was sentenced to a minimum of 14.
Ombudsman Dr Michael Maguire said Police Service of Northern Ireland (PSNI) decision-making which led to Hegarty's release was flawed, failed to protect his victims and to adequately assess the risk presented by a violent offender.
He added: "This was a man who was flagged on police systems as a violent offender and who had breached court bail conditions the previous weekend, when they believed he committed a violent assault.
"Police decided to release him to a different address, even though only a court can authorise a change of court bail address.
"They did not properly check the suitability of this new property: had they done so they would have established it did not have an electricity supply needed to operate the electronic tag."
Hegarty had a lengthy criminal record, with more than 70 convictions and several allegations of abuse of previous partners. He was known to police as a violent offender and wore the tag to monitor his movements at night.
The Ombudsman catalogued a series of flawed PSNI decisions, including an incident when police went to a house where, unknown to them, it's thought Hegarty was holding Ms Smyth against her will.
Officers left without checking the back door of the property after receiving no response to them knocking on the front door and window.
The Ombudsman's investigation began after the discovery of the bodies of Ms Smyth and Finbar McGrillen at a property in south Belfast on 13 December 2013.
Police held Hegarty in custody several times that year, including the weekend before the murders, when he allegedly assaulted Ms Smyth.
He had been given a prison sentence for assaulting a former partner.
An expert panel found he could cause serious harm through a sexual or violent offence.
But in May 2013 he was released from prison and began a relationship with Ms Smyth, from Drumaness in Co Down.
He was charged with another assault but released on bail.
On 7 December, officers called at Ms Smyth's address to conduct a bail check.
According to a report she later made, he was inside, holding her against her will.
Further checks on Hegarty's tag should have confirmed he was on the premises but the matter was not checked, the Ombudsman said.
Officers filed a report that he had breached his bail conditions but took no other action that day, Dr Maguire added.
The following day Ms Smyth told police Hegarty had locked her in the house since Friday and assaulted her with a metal bar. She said she managed to escape and had gone to a relative's house.
PSNI assistant chief constable Alan Todd said: "The PSNI is committed to keeping people safe.
"This report is difficult reading and I acknowledge that on this occasion we failed to effectively manage the risk posed to the public by Sean Hegarty and for that we apologise."