The police did not publish any "significant information" about a loyalist gun attack that killed five people, the police officer said.
The attack on Sean Graham's bookmaking business on Ormeau Road in southern Belfast in 1992 was carried out by the Ulster Freedom Fighters (UFF).
The Ombudsman has opened new lines of investigation into this attack, loyalist paramilitaries and a separate murder.
PSNI said she never tried to deliberately withhold the information.
The problem is due, among other things, to human error, "the sheer volume of material and the limitations of archaic IT systems".
Victims' families previously said that they believe that betting shop shootings have resulted in a collusion between the killers and the security forces.
Nobody was convicted of the murders.
The police ombudsman, dr. Michael Maguire said his staff were aware that the police were preparing to release material in a forthcoming civil case.
His office then asked for this material and it helped his staff identify "significant evidence relevant to a number of our investigations," he added.
"The police have now also identified a computer system that was not properly searched for prior requests for information," said Drs. Maguire.
"It seems that information was found that the police told us they did not exist."
The material prompted the Ombudsman of the Police to open new lines of inquiry into the Ormeau shootings, events involving loyalist paramilitaries in north-west Ireland between 1988 and 1994, and the assassination of young Damien Walsh at a coal depot in West Belfast in 1993 to investigate .
Reports from police officers on these investigations are now being postponed.
Dr. Maguire said, "in the interest of public trust in policing," he asked the Stormont Department of Justice to commission an independent review of the methods used by the police to disclose information.
The Judiciary Administration Committee (CAJ) said development has shown that "the practice (withholding information and delaying disclosure) is still ongoing."
"[It] is deeply shocking and the claim that it is based on human error simply offends our intelligence, "he added.
"The Office of the Police Ombudsman relies on the PSNI acting in good faith to assist in its investigation as RUC archive material is under its control."
"These developments clearly demonstrate PSNI's unwillingness or ability to bring the police officer fully informed so that he can conduct independent and effective investigations."
Tommy Duffin, whose father Jack was one of those killed in the attack, said it was an "absolute shame."
"It's a downer for the families … all we have is recoil after recoil, and this has almost broken the camel's back.
"We thought we would get good news from here."
PSNI Deputy Police Chief Stephen Martin apologized to the families of the victims of the attack.
"We deeply regret that the researchers who responded to the request of the Northern Ombudsman for Police (PONI) were unable to find and disclose them," he said.
The "mistake is obvious," he said, when a researcher working elsewhere in the PSNI "found the material as he prepared for civil litigation."
He said that there were a number of reasons why a researcher found the material, while others did not find it, including "different experiences and knowledge of our researchers."
Mr. Martin said that the PSNI Police Chief concluded that the best interim solution to public confidence in policing was to give the "Ombudsman of Police" "full and unrestricted access" to his legacy systems.
He also said that PSNI hopes that it will make significant changes to the information disclosure procedures in the coming months, and welcomes any independent review of its system.
A spokesman for members of the judiciary claimed that there was a "systemic problem regarding the disclosure of state killings and, in particular, killings where collusion is a feature".
Anne Connolly, head of the police department, said it was "essential" that the material was made fully accessible so that the office of the police officer could do his job. "
"While PSNI has outlined some of the complexities around the volume and location of material related to legacy research, the systems and verification processes do not meet the legal disclosure obligations and therefore need to be reviewed," she said.
In 2015, the police chief of the PSNI apologized when it was discovered that the weapon used in the Ormeau Road killing was displayed at the Imperial War Museum in London.
Patrick Shanaghan's family, killed in 1991 by loyalists near his home in County Tyrone, claimed that the recent revelations revealed failures at the heart of the police service.
In a statement, they said, "More delays, all because of constant collusion and cover-ups in our police service.
"What other conclusion can we draw from the Ombudsman's statement today, except that police still withhold information?"