SINN Féin are set to meet police after it emerged the PSNI failed to disclose "significant information" relating to the Sean Graham bookmakers killings.
Police Ombudsman Michael Maguire found that "significant, sensitive information" around the loyalist murders in south Belfast was not made available to his investigators.
Five people were killed on February 5 1992 when the UFF opened fire on the Sean Graham bookmakers shop on the lower Ormeau Road.
Dr Maguire's office said the material has opened new lines of inquiry in its investigation into the attack, as well as activities of loyalist paramilitaries in the north west between 1988 and 1994, and its probe into the murder of teenager Damien Walsh at a coal depot in west Belfast in 1993.
PSNI Deputy Chief Constable Stephen Martin apologised on behalf of the police and said they never sought to withhold the information from the ombudsman investigators, putting the incident down to human error.
On Monday, Sinn Féin leader Mary-Lou McDonald and deputy leader Michelle O'Neill are set to meet relatives of victims affected, the ombudsman and PSNI officers.
Mrs McDonald described the incident as "appalling and unacceptable".
"Many see these latest revelations as further evidence that the cover-up of the role of British state forces in the conflict in Ireland is systemic," she said.
"Confidence in policing is at its lowest level for many years. Action is needed."
On Saturday, Time for Truth campaigners, including family members of those killed in the Sean Graham shooting, took to the streets of Belfast to call for legacy mechanisms to be put in place.
They highlighted demands for the implementation of the mechanisms negotiated in the Stormont House Agreement, adequate funding of legacy inquests, and of the police ombudsman's office to allow it to complete outstanding historical investigations.
Mrs McDonald also called for the legacy structure to be implemented.
"The British government must immediately provide funding to the lord chief justice for legacy inquests and adequate funding to the ombudsman's office to properly discharge its duties without further hindrance to its investigations," she said.
"It is most important to remember that it is the families of the victims who are being let down once again.
"We will be speaking to families of the victims of the Ormeau Road bookies killings and other families hurt by this latest sickening blow.
"These families have been waiting 27 years for truth and justice."
