Relatives of the McGurk's Bar bombing victims have told a police watchdog that documents they have obtained contain evidence of security force collusion in the attack.
Family members of some of the 15 people killed in the 1971 loyalist blast in Belfast met with representatives of the Police Ombudsman's office to press for a new investigation.
The bombing was carried out by the UVF, but had initially been presented by the Royal Ulster Constabulary (RUC) as an accidental "own goal" by the IRA, prompting speculation that the dead might have included IRA members who were carrying the device.
A Police Ombudsman's report in 2011 said RUC officers had shown an "investigative bias" with the original misattribution of blame.
Bereaved relatives believe papers recently obtained in a trawl of historic records indicate the officers and others in authority were guilty of collusion.
They say the papers prove the bomb was planted in the doorway of the pub.
They claim the authorities therefore knew from early in the investigation that the bomb had been left at the door, rather than detonating in the possession of someone inside.
They contend that the subsequent portrayal of the bombing as an IRA own goal was an act of collusion with loyalists.
Robert McClenaghan, whose 73-year-old grandfather Philip Garry died in the bombing, said the ombudsman's office should open a fresh probe.
"We are here today to present this new evidence to the Police Ombudsman," he said.
"The Police Ombudsman can only reinvestigate McGurk's Bar if there is new evidence or if it is a very grave or exceptional set of circumstances. We believe that we fit those circumstances because we believe this is a very grave and exceptional situation that has developed since we found these new documents.
"We believe there is enough new evidence to warrant a finding of collusion."
Sinn Féin MLA Gerry Kelly and Nichola Mallon of the SDLP joined the families as they met Ombudsman Dr Michael Maguire in Belfast.
Afterwards, a spokesman for Mr Maguire described the meeting as "cordial and informative".
"The Police Ombudsman undertook to assess the documents provided by the families to determine whether or not any further investigative action is required by his office," he said.
"A further meeting will be held in the future after he has been able to obtain further clarity around the issues raised."