Police in Northern Ireland failed to disclose "significant information" relating to a notorious loyalist mass shooting to a police watchdog.
Ombudsman Michael Maguire has now contacted the Department of Justice to ask that the PSNI faces a review of how it discloses information.
Dr Maguire's office had found that "significant, sensitive information" around a mass shooting at a bookmakers' in south Belfast was not made available to his investigators.
Some of the information relates to covert policing, the Ombudsman's office added.
Five people were killed on February 5, 1992, when members of the loyalist Ulster Freedom Fighters opened fire on the Sean Graham bookmakers shop on the lower Ormeau Road.
A victims' organisation, Relatives for Justice, which represents the families of those killed in the atrocity has backed Dr Maguire's review call, adding it "should begin as a matter of urgency".
PSNI Deputy Chief Constable Stephen Martin has apologised on behalf of the police, and said they never sought to withhold the information from the ombudsman investigators, putting the incident down to human error.
He has also offered to give Ombudsman investigators "full and unfettered access" to police legacy systems.
The Ombudsman's office said the material in question has opened new lines of inquiry in its investigation into the Ormeau Road shootings as well as activities of loyalist paramilitaries in the north west between 1988 and 1994; and its investigation into the murder of teenager Damien Walsh at a coal depot in west Belfast in 1993.
Reports outlining the findings of these investigations, which had been due to be published in the coming weeks, will now be delayed.
Police Ombudsman Michael Maguire said it appeared that information which police told them did not exist has now been found.
"My staff became aware that police were preparing to disclose a range of material as part of impending civil proceedings," he said.
"Following a request from this office, police released this material to us which helped identify significant evidence relevant to a number of our investigations.
"Following on from this, police have now also identified a computer system which they say had not been properly searched when responding to previous requests for information.
Dr Maguire added: "It is right and proper that we examine the material which has now become available to ensure that our work provides as complete a picture as possible for these families, for the public and for the police.
"The public must have confidence that, when asked, police provide all the relevant information they hold on given matters, whether it be to this office or to other legal authorities."
Mr Martin said the PSNI "regularly and routinely discloses information" to the Ombudsman, and regrets that it was not done so in this case.
Mr Martin continued: "This error became apparent when, in line with our commitment to maximum transparency, a different researcher working elsewhere in the PSNI found the material while preparing for disclosure in response to civil litigation.
"The fact that one part of the organisation was able to find the information while the other did not is a result of a number of issues including the differing levels of experience and knowledge of our researchers, the sheer volume of the material involved and the limitations of the archaic IT systems.
"We entirely agree with the Police Ombudsman, that the effective disclosure of information is central to any system for dealing with the past."