Witnesses to the murder of Belfast man Robert McCartney are still being intimidated, his family has said.
Robert McCartney, 33, was killed near Belfast city centre
Mr McCartney, a 33-year-old father of two, was stabbed to death near Belfast city centre on 30 January following a row in a bar.
His sister Claire made the claim in response to Tuesday's IRA statement, which said they had offered to shoot those involved in his killing.
Detectives in Belfast are questioning a man in connection with the murder.
The McCartney family said it was only in court that "justice would be done".
The family claimed up to 12 IRA members were involved in the cover-up following the murder.
"It was the cover-up which prevented those who murdered Robert from being brought to justice," they said in a statement on Wednesday.
They said the IRA could not give any reason why Robert was killed when the family met the IRA on 5 March.
"It is now five weeks since Robert was murdered and no-one has come forward with substantial evidence. This must be due to ongoing intimidation and fear," the family said.
The family, who will travel to Washington to meet President Bush next week, are now considering whether to open a campaign office.
In an interview with the BBC, his sister Catherine said it was up to the republican movement to work out how to place the people who killed Mr McCartney before a court.
"People should not be asking us 'what do you want the IRA to do, what do you want Sinn Fein to do?'," she said.
"I do not believe the IRA, who, remember are nearly 100 years old and in their history are probably one of the most notorious guerilla warfare machines in the world, cannot resolve this issue."
On Wednesday evening, police officers posted fresh appeals in the Short Strand area where Mr McCartney lived for people with information to come forward.
Last week, police said that 10 people previously arrested over the killing stayed silent throughout questioning. All were released without charge.
On Tuesday, the IRA said it had offered to shoot the people it says killed the father-of-two. His family has rejected the offer.
'Prepared to kill'
Chief Constable Hugh Orde has said he is certain the IRA's statement offering to shoot those behind the murder meant it was prepared to kill them.
He said: "This is an organisation theoretically on ceasefire. This is an organisation that is still prepared to kill people now from its own community".
Meanwhile, Prime Minister Tony Blair told the House of Commons the IRA statement was "extraordinary".
"It cannot be in any shape or form justified," Mr Blair said.
Sinn Fein's Martin McGuinness said he was surprised by the IRA statement.
He said he thought it would have been "very unfortunate" if the organisation had shot the alleged killers.
Irish Premier Bertie Ahern said the IRA statement was "extraordinary and horrific".
Mr Ahern told a news conference in Dublin he had been shocked by the comment, but that things had to move forward in the peace process and there was a distinction between Sinn Fein and the IRA.
The US Special Envoy to Northern Ireland, Mitchell Reiss, said it was "time for the IRA to go out of business".
Mr Reiss added: "It's time for Sinn Fein to be able to say explicitly, without ambiguity, without ambivalence, that criminality will not be tolerated."