The PSNI has acknowledged deficiencies in its investigation into the murder of 21-year-old showjumper Katie Simpson.
Assistant Chief Constable Davy Beck said "no family should ever have to experience" what Ms Simpson's family are going through, and apologised for the shortcomings during the investigation into her death.
In fact, she had been strangled and beaten by her sister's boyfriend Jonathan Creswell.
The investigation became a murder inquiry after it was turned over to a different team of detectives.
Creswell was found dead at his home as the second day of his murder trial was about to get under way.
Six officers were recommended for disciplinary action by the police ombudsman.
One got a written warning. Two others get lesser sanctions and in one case there was no finding of misconduct.
Another two officers left the PSNI, which brought the cases against them to a halt.
The Simpson family said they believed there’d been "buck passing" with the blame pinned on more junior officers when the focus should have been higher up the chain of command.
Three sergeants were the ones who were disciplined.
At police headquarters, Mr Beck addressed the deficiencies in the original investigation.
"I acknowledge that there were shortcomings in various stages of the investigation into Katie’s death and I unreservedly apologise for this.
"It is clear that we were not rigorous enough in our pursuit of all potential lines of enquiry and did not act quickly enough in responding to some of the concerns that were raised around Katie’s death," Mr Beck said.
He added: "We relied too much on the lack of any presenting medical evidence around what had happened to Katie when she was first admitted to hospital and on the forensic post mortem that was conducted on 11 August 2020.
"Our systems failed to identify Creswell’s previous related, serious offending. We acknowledge this and reaffirm our position that we must and will do better, moving forward."
Assistant Chief Constable Beck also said he is committed to responding to concerns for the safety of women and girls.
"We let Katie's family down in this investigation. We don't want to let other women and girls down."
He added: "I am committed to learning from this and ensuring the PSNI do better in response to these failings."
Mr Beck said the PSNI will ensure that officers have the confidence to investigate fully offences like this.
"We were too accepting of Creswell's first account.
"We did not engage widely enough and early enough with the family and friends of Katie Simpson and perhaps if we had, we would have unpicked a much earlier stage the shortcomings in his account."
He said "no one person was responsible for that", but he said as an organisation the PSNI were responsible for these failings.
The Ombudsman’s report was "important work", Mr Beck said, in order to reassure the public that cases like these will be handled correctly and thoroughly.
Mr Beck referred to what he called "gaps" in legislation at the time of Ms Simpson's death which he said have "thankfully now been filled".
He added that new laws are now in place to deal with crimes like non-fatal strangulation, coercive control, domestic violence and stalking.
"I am more assured now that there is less opportunity for us to miss potential offending because the legislation allows us to take more swift and appropriate action."
Additional reporting Carla O'Brien
