Sinn Fein must use all its influence to make sure the killers of Robert McCartney "have nowhere to hide", his family has said.
Robert McCartney, 33, was murdered in Belfast city centre
Mr McCartney, 33, was stabbed after a pub fight in Belfast two weeks ago.
His family has said republicans were pressuring witnesses not to talk to the police about the murder.
Sinn Fein Assembly member for North Belfast Gerry Kelly said he accepted the family's belief that some witnesses in the case had been intimidated.
Mr Kelly, who stopped short of urging people to go to the police, said he wanted the same thing as the family - and that was "justice for Robert McCartney".
After meeting the US consul general on Wednesday, the family welcomed "belated comments" by Gerry Adams urging people to give information about the killing.
The McCartney family said they had a good meeting with consul Dean Pittman in Belfast and said he had pledged to raise the case at higher levels in the US administration.
The murder victim's sister Claire McCartney said: "We are in full agreement that those involved did not 'act like republicans or on behalf of republicans'.
"However, the call for those with information to go to a solicitor, priest or ourselves raises questions such as what would this achieve? Would this bring Robert's murderers to justice? Would this secure convictions?
"We call upon Sinn Fein to bring all its influence to bear on those who have the power to ensure that these individuals are left with nowhere to hide.
"We are aware of Sinn Fein's policy on the police, but Sinn Fein, as other political parties, are capable of not being seen to take a certain course of action.
"These individuals must face justice if we are to move on from Robert's death. The community needs to feel safe and free also from these psychopaths."
Lines of inquiry
Mr Adams joined appeals for people to give information about the killing.
He says if people do not feel comfortable talking to the police they should pass on information to the family or a solicitor.
However, SDLP MLA Alban Maginness, a barrister, said it was nonsense for Sinn Fein to suggest this as lawyers were bound by confidentiality.
"They are trying to evade or to avoid telling police what actually happened and they want people to really enter into this quasi legal charade," he said.
The police have said almost 500 lines of inquiry were being followed by detectives investigating the murder.
They also said a large quantity of CCTV pictures were being examined.
However, detectives have refused to comment on claims that a tape was missing from the bar where the fight took place.
The murder weapon has not yet been found.
United States consul general Dean Pittman is meeting members of Mr McCartney's family on Wednesday.