Journalist and Bloody Sunday campaigner Eamon McCann today urged the Provisional IRA Army Council to release members from their oath of secrecy to testify to the Tribunal into the killings.
Issuing the appeal from the witness box at the Saville Inquiry, he also said he now believed the terror group fired no shots in Derry on January 30 1972 when British army paratroopers shot dead 13 Catholic men in the city.
His claim contradicted allegations by an MI5 agent last year that Sinn Fein's Martin McGuinness fired the first shot of the day, sparking the tragic chain of events that day.
Mr McCann has said he no longer believes Mr McGuinness - now the Mid Ulster MP and Northern Ireland Education Minister - was the OC of the Provisional IRA at the time, as was also alleged by the agent, known only as "Infliction".
Five members of the former Official IRA - the Provisionals' precursor - came forward last month and are currently seeking anonymity in order to testify but repeated appeals to the Provisionals have largely met with silence.
Tribunal chairman Lord Saville of Newdigate defended the approach of Inquiry lawyers in asking unconnected civilians what they knew about the IRA at the time following claims that it was arousing increasing resentment in Derry.
Lord Saville said: "It is our basic duty to discover the whole truth. If the only way we can do that, however unsatisfactory though it is, is to ask questions about rumours and who knew who and so on, we are forced to adopt that method.
"It is not a satisfactory method but we have a duty to do to, we cannot just let that slide away because people are not prepared to help us.
"If they are not prepared to help us we must do what we can ourselves to discover who they are and whether they have information which leads to the whole truth about Bloody Sunday."
Later, Mr McCann said the only people who could provide the Tribunal with accurate information about what the IRA did on Bloody Sunday were those in the organisation itself.
A member of the Bloody Sunday Trust, which has campaigned on behalf of the bereaved and wounded, he added: "The families are very aware of the fact that if members of, for example, the Provisional IRA do not come forward and give their account of what they were doing on Bloody Sunday, then it is inevitable that inferences adverse to them might be drawn.
"I think it is the feeling of the families that for as long as IRA members do not come forward, then they are in effect polluting the truth about Bloody Sunday by not clearing up what they were doing."
Under examination from Arthur Harvey QC, acting for most of the next-of-kin, Mr McCann said he was "reasonably satisfied" the Official IRA members who fired shots on Bloody Sunday and their commanders would come forward to assist the tribunal.
But he believed a problem facing the Provisional wing was members' oath of secrecy, a position regarded with "deadly seriousness".
"I believe they would be greatly facilitated in coming forward if the current army council of the Provisional IRA were to release them, as it were, from their solemn undertakings of secrecy," he said.
It would be an unprecedented move - but the organisation had taken other unprecedented decisions in recent years, he said.
Mr Harvey, who last month issued an appeal on behalf of all the bereaved and wounded for anyone with information about Bloody Sunday "however unpalatable" to come forward, said there appeared to be at least five and possibly six "civilians" who fired shots in Derry that day.
He asked Mr McCann if he believed any of those were fired by members of the Provisionals, to the reply: "No, it is my belief now that the Provisional IRA did not fire any weapons on Bloody Sunday."
He agreed there was a sense of outrage in the community that there had been any gunfire that day because it gave apparent justification to the British army for the deaths of innocent people.
But he added: "I believe that the overwhelming majority, if not every single person in the Bogside and Creggan community, believes that nothing that was done by republicans on the day materially affected the pattern of events or could rationally be construed as having provoked the killing and wounding of 27 people.
"It is one of the reasons why many people are very anxious that republicans will come to this tribunal and say exactly what they did and what their intentions were on the day because it is difficult to establish that if they do not come forward."
The Inquiry resumes tomorrow.