President George Bush has said he has invited the family of Robert McCartney to meet him as part of a thank you to those working towards peace.
The McCartney sisters held talks with Senator Ted Kennedy
Relatives of the Belfast man, who was stabbed to death, will meet the American president on St Patrick's Day.
Mr Bush added: "I am looking forward to meeting these very brave souls."
The US Special Envoy to Northern Ireland, Mitchell Reiss, said the US administration was doing all in its power to help the family.
Mr Reiss was speaking after he met the McCartney family in Washington on Wednesday to discuss the "terrible, terrible murder".
Mr McCartney, 33, was murdered following a row in a Belfast bar on 30 January.
Mr Reiss said he stood with all Americans in his admiration for the "courage and determination" of Mr McCartney's family.
The McCartney family also held talks with US Senator Ted Kennedy, who has refused to meet Gerry Adams during the Sinn Fein leader's St Patrick's week trip to the US.
A spokeswoman for Senator Kennedy said he had cancelled a meeting because of the IRA's "ongoing criminal activity".
Speaking after meeting the family, Senator Kennedy said their presence in Washington "sends a very powerful signal that it's time for the IRA to fully decommission, end all criminal activity and cease to exist as a paramilitary organisation."
MCCARTNEYS' US SCHEDULE
Wednesday 10am EST: State Department meeting with US envoy to NI
11am: Meeting Senator Ted Kennedy
7pm: Gala dinner organised by American/Ireland Fund
Thursday 9.30am: arrival at White House for St Patrick's Day reception
He told a news conference: "No political party can also have an armed unit that continues violence and criminality.
"We would certainly hope that the leadership of Sinn Fein... understands what an albatross the IRA is on them and for the cause of peace in Ireland."
However, Sinn Fein President Gerry Adams later hit back at the comments, saying Senator Kennedy's remarks were not helpful.
"I have already set out very clearly Sinn Fein's efforts to create the
conditions in which the IRA ceases to be. It is my conviction that we
will be successful," he said.
The meeting was also attended by Senator Hillary Clinton and former US presidential hopeful Senator John McCain.
They each said they were inspired by the way the McCartney family had stood up to the IRA to demand justice.
Catherine McCartney said they knew before their visit that they would have the senators' support.
"We hope that this does produce results on the ground for this family," she said.
Robert McCartney, 33, was killed near Belfast city centre
"Because until it does we cannot move on. We cannot rebuild our lives until these people are brought to justice."
Northern Ireland Secretary Paul Murphy, who is in Washington to attend St Patrick's Day events, said it was clear the intensely personal way the McCartneys had presented their case was having a deep impact.
"It is certainly playing a part in the peace process," he said. "You can see already here in the US what has happened in just two days.
"Everybody understands that the McCartneys' intent symbolises the difficulty that we are facing in Northern Ireland."
The five McCartney sisters and Mr McCartney's fiancee, Bridgeen Hagans, will meet President Bush on Thursday and plan to give him a dossier on the events before and after the murder.
Gerry Adams has been denied a meeting with President Bush on St Patrick's Day, the same day on which he had been due to meet Mr Kennedy.
The McCartney family's visit to America comes after Sinn Fein criticised the handling of the murder inquiry by police in Northern Ireland.
Martin McGuinness accused the police of "unprecedented and incredible delays" in questioning key suspects and witnesses just to damage his party.
Three men were expelled by the IRA after the killing, including the two main suspects in the case.