Alleged child abuse victim Mairia Cahill was "failed" by a disjointed PSNI investigation, a watchdog has said.
In 2010 Ms Cahill, a grand-niece of prominent Belfast republican Joe Cahill, told officers she had been sexually abused by alleged IRA member Martin Morris from 1997 to 1998.
Morris was acquitted of rape and denies all wrongdoing.
In subsequent years she said she was subjected to an IRA interrogation over her allegations.
Police ombudsman Dr Michael Maguire has said four officers should be disciplined over shortcomings in the police response.
The PSNI's Chief Constable has said he accepts the report and has apologised 'unequivocally' for the hurt and distress caused to victims.
George Hamilton said the reommendations made by the ombudsman have since been implemented.
Dr Maguire criticised the force's decision not to hold a serious case review and the circumstances of the choice to split its investigation across two units: one with expertise in terrorist cases and another specialised in dealing with victims of sexual assault.
He added: "I accept that police wanted to move quickly on the sexual allegations and to use their different expertise to maximum effect.
"While I do not agree that this led to evidence being diluted, it did bring about a disjointed approach by police in their investigations and their treatment of Ms Cahill.
"There is no evidence they considered any other approach, such as creating a team with the range of skills to investigate these matters as one case."
Ms Cahill, then aged 16, had told police she was subjected to a 12-month ordeal of sexual abuse.
She alleged republican paramilitaries conducted their own inquiry and subjected her to interrogation before forcing her to confront her alleged attacker.
Her allegations, highlighted in a BBC documentary, shone a light on how the IRA dealt with alleged sex abusers during a time when co-operation with the police in republican communities was extremely limited.
The attempted prosecutions of Mr Morris for alleged sex abuse and IRA membership - and four others accused of IRA membership linked to Ms Cahill's claims of a republican cover-up - never got to trial because three women withdrew their evidence.
A review by the former DPP in England and Wales, Sir Keir Starmer, said the Northern Ireland Public Prosecution Service's (PPS) handling of what were planned to be three separate trials had "let down" the three women.
The Ombudsman's office said: "Overall, the Police Ombudsman found that the PSNI investigation had failed the victims, but he did not support the allegations that it chose not to arrest some of the individuals concerned because they were police informants and that it had been subject to political interference."
Those failings included the PSNI's "inconsistent" approach in its investigation of some of the people suspected of IRA membership, which in one case led to an individual not being arrested and questioned.
The Ombudsman found no evidence that anyone had been protected from prosecution or that the PSNI investigation became subject to adverse political interference.
His investigation did not find that Ms Cahill had to direct how the investigation progressed, but said the force's lack of a strategy for researching information already in the public domain contributed to her mounting concerns.
Three of the officers recommended for action have been disciplined. The fourth had retired. Dr Maguire also made recommendations for changes to PSNI policies.
Sinn Féin President Mary Lou McDonald has welcomed the publication of the report and has apologised that the party did not have procedures in place for mandatory reporting of abuse at the time.
Mairia Cahill had demanded an apology from Ms McDonald in the wake of report.
Ms McDonald said: “I welcome the publication of the Ombudsman’s report and the fact that the PSNI have accepted and will implement the recommendations of that report.
“Abuse has scarred too many lives across Ireland. We all have a responsibility to keep children safe.
“I have no doubt that the three women at the heart of this report have been through an ordeal.
“I want to commend their bravery, in particular the bravery of Mairia Cahill for waiving her anonymity.
“Sinn Féin has robust procedures in place for mandatory reporting of abuse.
“I deeply regret that these procedures were not in place at the time of Mairia Cahill’s disclosure. For this I unreservedly apologise.
“I wish Mairia Cahill every best wish for the future.”
Ms Cahill slammed the apology, calling it "woeful".
- Watch Sara Moore's report: