The PSNI can never "fully eradicate" the risk that it will fail to find and disclose some records it holds about the Troubles, a report has concluded.
The Criminal Justice Inspection (CJI) found the service had "no central inventory" of all historical records.
The review was ordered after the PSNI did not initially hand over all relevant papers to a police ombudsman investigation.
Its staff were examining a loyalist gun attack in 1992.
The PSNI holds 44 million document items relating to three decades of violence.
The CJI report was undertaken after it was discovered police "human error" meant "significant" material was not given to assist a Police Ombudsman inquiry.
The ombudsman is investigating the UFF murders of five Catholics in a bookmakers shop on the Ormeau Road in Belfast in 1992.
The CJI's Jacqui Durkan said the PSNI needed to carry out a full audit of its records, held in several different locations.
She added the PSNI and ombudsman should "work hard together" to strengthen trust and repair public confidence.
Relatives for Justice, which represents some of the victims' families, said the PSNI had treated the ombudsman with "utter contempt".
It alleged there was a culture within the police to defend its predecessor, the Royal Ulster Constabulary, from scrutiny.
PSNI Deputy Chief Constable Mark Hamilton said: "The inspectorate are right when they say the risk of a further error cannot be fully eradicated.
"That is why we must continue to critically evaluate our systems and processes to limit the potential for any recurrence."