Sinn Féin president Mr Gerry Adams last night urged anyone who knew about Mr Robert McCartney's killing to pass that information on.
The West Belfast MP also launched a scathing attack on those who carried out the stabbing and stressed that his party supported the victim's family in their search for justice.
"There are allegations that Robert McCartney was killed by republicans," he said. "I want to make it absolutely clear that no one involved acted as a republican or on behalf of republicans.
"I repudiate this brutal killing in the strongest terms possible. No one has any right, as has been claimed, to prevent anyone from helping the McCartney family.
"People with reservations about assisting the PSNI should give any information they might have either to the family, a solicitor or any other authoritative or reputable person or body."
Earlier yesterday, Sinn Féin's chief negotiator Mr Martin McGuinness said he would not criticise anyone who gave information "to the appropriate authorities" about the £26 million Northern Bank robbery in Belfast last December. He also said he was satisfied that no breakaway group within the Provisional IRA carried out the robbery which he declined to describe as a crime.
Interviewed on BBC Radio Foyle yesterday morning, Mr McGuinness repeated his assertion that those responsible for the robbery had damaged the peace process and added that his firm belief was that the Provisional IRA was not responsible.
"No I don't believe they did it," he said. "I was very conscientious about first of all going to the IRA and establishing from them had they done it, and trying to establish the possibility that someone in the ranks of the IRA would have been involved because there were of course previous occasions, we had the Garda McCabe situation, and I thought it was absolutely essential, to the best of my ability, to rule out any prospect that that could have happened.
"I am satisfied as the result of those conversations that the IRA were not in any way involved in the Northern Bank robbery."
He said he believed "without exception" that it was "absolutely wrong" for any organisation to be involved in actions, including bank robberies, which could undermine the peace process.
Asked would it be an act of criminality if it was proven there was IRA involvement in the robbery, Mr McGuinness said that to "ask me to go down the road of this huge discussion of criminality is to then ask me to adjudicate on every single action that occurred over 25 years of conflict, and what is at the heart of this debate is to try to get Sinn Féin leaders to say that the IRA are criminals.
"I am absolutely opposed to criminality. I have a very clear view that anyone involved in crime should be arrested, they should be brought before the courts and given due process."
Asked if they should be arrested by the PSNI, he said they should be arrested "by whoever the appropriate authorities are. In my own constituency in Mid-Ulster, a 75-year-old man was murdered over the Christmas period. He was smothered in his bed . . . I told the local community that they should not hold back any information they had and that they should give that information to whoever they deemed to be the appropriate authorities."