The Police Service of Northern Ireland is facing a barrage of criticism and questions for failing to disclose to a police watchdog “significant” information about loyalist paramilitary murders during the Troubles.

The head of the Police Ombudsman for Northern Ireland, Michael Maguire, on Wednesday asked the Department of Justice for an independent review into why the force did not share information about a mass shooting of five Catholics at a betting shop in Belfast on 5 February 1992.

The PSNI has apologised and blamed human error, citing “complex challenges associated with voluminous material”. It denied deliberately withholding information.

The release of the information has prompted the ombudsman to pursue new lines of inquiry into about 20 loyalist murders across Northern Ireland and the Irish Republic in the 1980s and 1990s. Ombudsman reports into those killings, which were expected in coming weeks, have been delayed.

“It would seem information which police told us did not exist has now been found,” said Maguire.

The ombudsman’s office learned of the information when police prepared to disclose it to relatives of those killed in the 1992 attack as part of civil proceedings.

“Following a request from this office police released this material to us which helped identify significant evidence relevant to a number of our investigations,” said Maguire. “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.”

The ombudsman called for an independent review in the interests of “public confidence” into a force set up after the 1998 Good Friday agreement, in the hope the force would win more support from Catholics and nationalists than its predecessor, the Royal Ulster Constabulary.

The PSNI responded swiftly on Wednesday with an apology and promise to overhaul the way it disclosed information.

“We deeply regret that the researchers responding to the Police Ombudsman for Northern Ireland’s request were unable to find and disclose it,” said the deputy chief constable, Stephen Martin. The varying levels of experience and knowledge of researchers accounted for the ombudsman receiving incomplete information but ombudsman staff would now receive “full and unfettered access” to material relating to the crimes, he said.

But groups representing victims expressed dismay and said the incident revived concerns about police collusion with loyalist paramilitaries. The Committee on the Administration of Justice said: “[It] is deeply shocking and the claim that it is due to human error simply insults our intelligence.”

Another group, Relatives for Justice, said there was a systemic problem in disclosure of details about killings involving collusion. It said the independent review should start as a matter of urgency.

Tommy Duffin, whose father, Jack, was one of those killed in the attack at the Sean Graham betting shop, on Ormeau Road, told the BBC that relatives were frustrated by the decades’ long quest to uncover details about the massacre. “All we have got is knock-back after knock-back, and this has nearly broken the camel’s back.”

Sinn Féin requested an urgent meeting with police chiefs to discuss what it termed an “appalling and unacceptable” failure.