The PSNI has apologised for its failure to provide Northern Ireland's Police Ombudsman with all the available information while he was investigating controversial killings carried out by loyalist paramilitaries in the 1990s.
Some of the information relates to covert policing, the ombudsman's office has said.
The cases that ombudsman Dr Michael Maguire was investigating included the Halloween killing of eight people at a pub in Greysteel, Co Derry in 1993; the killing of five people at a betting shop on Belfast's Ormeau Road in February 1992, and the murder of Sinn Féin Councillor Eddie Fullerton in Buncrana, Co Donegal in May 1991.
Loyalist paramilitaries were involved in all the attacks.
Dr Maguire was finalising his reports and was on course to publish them before he retires from his post in the summer, but independent of his work, a relative of one of those killed in the Ormeau Road attack was preparing a civil action against the security services.
In response to those proceedings, the PSNI provided material that the ombudsman had not received.
As a result of the material that has emerged, the ombudsman is to revisit his earlier work and the publication of his report will now be delayed.
The PSNI said that human error and systems failings are responsible for the disclosure shortcomings and has apologised to the families affected.
Dr Maguire said the material helped his investigators to identify "significant evidence" in relation to a number of cases.
"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.
"In that instance, it would seem information which police told us did not exist has now been found."
Additional reporting PA