The Sinn Féin ardfheis in Dublin last night endorsed an emergency motion tabled by the party's justice spokesman Gerry Kelly deploring the "brutal and criminal" killing of Robert McCartney.
It also called on those responsible to make themselves accountable to the courts, and urged anyone with information to make it available "through whatever channel they deem appropriate".
The murder of Robert McCartney, the Northern Bank robbery and the IRA multimillion money-laundering claims overshadowed the ardfheis last night.
Sinn Féin's chief negotiator Martin McGuinness told delegates that what anguished him most was that republicanism could be diminished by allegations of criminality.
Sinn Féin president Gerry Adams is likely to return to this theme in his keynote speech, which will be broadcast on RTÉ this evening.
Mr McGuinness spent a considerable portion of his address in the RDS, Dublin, insisting that republicans must not be tarnished by criminality.
"What pains me the most about the last few weeks is not the criticism from the two governments, our political opponents, the media and those unionists who are so clearly delighted to have an excuse for their intransigence.
"We are used to that and we can take it," said Mr McGuinness.
"What pains me the most is any suggestion, suspicion or indication that the IRA could be turned into a criminal gang or a tool of individual interest, or otherwise engaged in criminality. And in that context I am both outraged and saddened at the involvement of a small number of IRA volunteers in the brutal killing of Robert McCartney," he added.
Mr McGuinness continued: "So let me be clear. The murder of Robert McCartney was wrong - and let me be absolutely clear this was a grievous crime. It is wrong, it should never have happened and it is wrong that those who witnessed the murder should be intimidated in any way.
"And the McCartney family are absolutely right when they say that those responsible should be held accountable for their actions and should make themselves accountable for their actions.
"I again urge all of those involved in any way to admit their role and to make themselves accountable in court.
"We cannot allow republicanism to be diminished in this way.
"To do so would be a betrayal of our struggle, of our own personal commitment, of the hunger strikers and of those brave republicans who selflessly gave their lives and liberty for a noble and worthy cause."
Mr McGuinness said there were lessons to be learned. "Painful as it is, we as republicans have to face the reality that there is a crisis of confidence that could destroy the Good Friday agreement," he told delegates.
"In December we got close to a historic agreement that would have put violence behind all of us forever.
"We were cheated only by the insistence of the unionists on the humiliation of the IRA," said Mr McGuinness.
"I am not prepared to let our struggle be demonised, or to be caught in a downward spiral that leads inexorably to a return to violence."
Mr McGuinness, who said Sinn Féin was determined the peace process would not fail, accused southern parties of deflecting attention away from injustice, inequality and failures of government by spreading "innuendo, smear and false allegations against Sinn Féin".
A critical debate on policing, which will indicate whether or not Mr Adams and the Sinn Féin leadership are prepared to begin steering republicans towards accepting the PSNI, takes place tomorrow.
The downturn in Sinn Féin's fortunes caused by the criminality claims and reflected in The Irish Times/TNSmrbi poll was not directly referred to by Mr McGuinness, who was the main speaker last night.
But his acknowledgement of how "very difficult" recent weeks have been for republicans indicated how that reversal is foremost in the consciousness of the Sinn Féin leadership.
Looking ahead, Mr McGuinness predicted strong gains in next week's byelections in the Republic and in the elections in the North in May.
