For a second time, the Police Ombudsman is to delay publication of three reports on loyalist murders after the emergence of additional documentation.
The material was found during new PSNI searches of its Troubles archives, coinciding with a recent IT upgrade.
Assistant Chief Constable George Clarke said he realised "how frustrating and upsetting" this was for the families involved.
One report covers the attack at Sean Graham bookmakers in Belfast in 1992.
The Police Ombudsman was made aware of the development about three weeks ago, and has so far assessed about 70% of "a considerable amount of information".
"To date we do not see anything which would cause concern or lead us to commence new lines of inquiry," Marie Anderson told BBC News NI.
'Not adequately resourced'
It is the second time the PSNI has found material related to the reports which was not disclosed initially.
In February the force apologised after finding "significant information" on computer files which had been overlooked.
An internal review was launched, with improvements made to its systems, and as a result a further volume of material has now been uncovered.
Families fear police have are attempting to "hold off on the truth until the families have passed away", according to Belfast solicitor Niall Murphy.
He said: "Families have a legitimate expectation that the Police Ombudsman will have unfettered access to all sensitive documentation.
"The fact is that has not happened. That is a matter of great distress to the families today.
"From the families' perspective, it's distressing, disappointing but not surprising".
The development has also impacted on an independent investigation ordered by the Department of Justice.
After February's incident, it asked the Criminal Justice Inspectorate to look into the effectiveness of PSNI procedures.
But publication of the report "has been suspended".
The Policing Board has been made aware of developments and will raise it with chief constable Simon Byrne at its November meeting.
As well as bookmakers attack, the ombudsman is also preparing reports on loyalist paramilitary activities in the north west and the murder of Damien Walsh in Belfast in 1993.
The PSNI has previously disclosed it holds more than 44 million pages of paper and microfilm related to the Troubles.
It has stated it is "not adequately resourced" to deal with legacy issues.