The PSNI has apologised over its failure to disclose "significant information" on a Troubles massacre to a police watchdog.
The discovery was made as part of an investigation into a shooting at a bookmakers in Belfast in 1992, during which five people were killed.
The attack was carried by loyalist paramilitaries.
A PSNI researcher found additional documents related to the case by using a different search term for weapons used during the Troubles.
The documents were revealed as part of civil proceedings related to the case, and were then submitted to the Ombudsman after the 'anomaly' was discovered.
Police in the North have blamed human error, the 'immense' volume of documents and 'archaic' computer systems for the failed disclosure.
"Deeply and sincerely sorry"
PSNI Deputy Chief Constable Stephen Martin said: "As set out by the Ombudsman in his public statement, we are now aware that information that should have been disclosed to his office was not; and we sincerely regret this.
“PSNI never sought to deliberately withhold this information from PONI (Office of the Police Ombudsman) and we deeply regret that the researchers responding to the PONI request were unable to find and disclose it."
He apologised to the families affected, and added that the force was "deeply and sincerely sorry".
The force is now pledging to give the Ombudsman "full and unfettered access" to PSNI legacy systems.
An independent review into the incident has been requested.
Ombudsman Dr Michael Maguire said the information was so valuable that it has opened new lines of inquiry into the killings.
He explained: "It's significant enough for me to not publish reports which were nearing completion.
"What I've said is that I need further investigative steps to be done before we'll be in a position to publish those reports. That's the unfortunate message I had to deliver to families."