The PSNI should immediately review its appointment and training of staffresponsible for disclosing information, an inspection report has concluded.
The service failed to produce all its files on the Sean Graham Bookmakers killings in Belfast in 1992.
Further errors cannot be ruled out, a review concluded.
Five people were killed by UDA paramilitaries in the gun attack.
The Criminal Justice Inspectorate said no specific guidance was available toofficers searching for material for a Police Ombudsman probe into how themurders were investigated.
It also praised the knowledge and professionalism of staff and said efforts had been made to fill recruitment gaps.
Chief Inspector of Criminal Justice in Northern Ireland Jacqui Durkin said:"Staffing increases and the transfer of information from outdated computersystems to modern IT platforms with better search capability are welcomedevelopments.
"...further work is needed by both organisations to establish more effective systems and better processes.
"However, further work is needed by both organisations to establish moreeffective systems and better processes."
In February last year the former Ombudsman said investigators had identified sensitive PSNI material which had not been made available in relation to the Sean Graham killings.
The PSNI acknowledged disclosure failings, apologised to those affected, and said that it had not sought to deliberately withhold information.
One part of the PSNI, the Legacy Support Unit, found material for a civillitigation which was not disclosed by another part of the organisation, theLiaison Office.
That showed that the PSNI needed a mechanism for making material available across the organisation, the inspector said.
Ms Durkin noted tensions between aspects of the independent Ombudsman and the PSNI's operations.
Those included the watchdog's repeated suspicions about less than fulldisclosure by the PSNI and potential harm to their relationship caused by large numbers of files requested by the Ombudsman then not read due to shifting priorities and other legacy work.
Police are introducing a new case management system to provide assurances that the force's disclosure and discovery regimes are effective and consistent across the organisation.
Ms Durkin urged the two organisations to 'work hard together' to strengthentrust and repair damage to public confidence.
She said: "Inspectors found substantial work was already under way within the PSNI to address issues that contributed to the initial police failure to provide all relevant material to the Ombudsman's office, when fieldwork for this review began in spring 2019."
That included staffing increases and the transfer of information from outdated systems to modern platforms with better search capability but further work was needed.
Ms Durkin said police needed to carry out a full audit of historical records toestablish what material it had and where it was located because there was no central inventory of hard copy historical material.
PSNI deputy chief constable Mark Hamilton said much of the work on therecommendations from the report is already well progressed and, in some areas, has already been completed, significantly reducing the risk of the error recurring.
"The Inspectorate are however right when they say that the risk of a furthererror cannot be fully eradicated and that is why, along with OPONI, we mustcontinue to critically evaluate our systems and processes to limit the potential for any recurrence."