Special Branch withheld information from detectives investigating a murder 15 years ago in Newry, County Down, the NI Police Ombudsman has found.
The Police Ombudsman's office investigated the 1990 murder
However, it was found there was no evidence the police colluded in the 1990 shooting of Eoin Morley.
His family alleged police colluded in the murder to provoke a republican feud between the IRA and the IPLO.
Nuala O'Loan's office said they found no evidence that the RUC had instigated the murder or knew who carried it out.
The family had alleged the RUC knew who was responsible but had sought to protect that person.
The ombudsman's office found significant failings in the murder inquiry, with Special Branch withholding information which may have had some relevance.
Mr Morley, 23, was shot dead in the Derrybeg area of Newry, County Down, on Easter Sunday, 15 April 1990.
The killing came amid tensions between the IRA and the Irish People's Liberation Organisation, a splinter republican terror faction.
Mrs O'Loan said no disciplinary action could be taken because the officers involved were either retired or through a lack of evidence against those still serving in the police.
Speaking on Friday, Mrs O'Loan said things which should have been done were not.
"I cannot criticise the investigator for not using the intelligence if the investigator is not given it," she said.
In a statement, the Police Service of Northern Ireland said there had been wide-ranging reforms since then, ensuring better sharing of information in the future.
The PSNI replaced the old Royal Ulster Constabulary as part of wide-ranging reforms of policing in Northern Ireland, recommended in the Patten Report.