The DUP is to raise with British prime minister Gordon Brown its concerns over the possible involvement of republicans in the murder of Paul Quinn.
Mr Quinn (21), from Cullyhanna in south Armagh, was beaten to death last month by a gang of men at a remote farm on the Co Monaghan side of the Border.
His family has blamed the Provisional IRA.
The DUP expressed fresh concerns yesterday following a meeting last week between senior representatives of the party and PSNI chief constable, Sir Hugh Orde.
Jeffrey Donaldson, who attended the meeting with party leader Dr Ian Paisley and deputy leader Peter Robinson, said Sir Hugh informed them there was evidence of republican involvement in the murder.
The meeting was arranged following a statement by a member of the Independent Monitoring Commission that members, former members or associates of the IRA were involved in Mr Quinn's killing.
"The chief constable has told us that some of the individuals involved in the brutal murder of Paul Quinn have associations with the IRA," Mr Donaldson told The Irish Times.
"What is not clear is whether they are operating at present as dissident republicans or whether as a criminal gang.
"The chief constable did confirm that neither the PSNI nor the security services have any evidence to suggest that this murder was authorised by the IRA at leadership level," he said.
"However, given this further development and the recent comments by the IMC we continue to have concerns and we will be discussing them with the IMC and the prime minister, Gordon Brown.
"We need to be absolutely clear that when the republican leadership say they are co-operating with the police that everything possible is being done to ensure that those responsible for this murder are brought to justice."
Mr Donaldson said the rule of law had to apply as much in south Armagh as anywhere else.
Dr Paisley, Mr Robinson and Mr Donaldson are members of the Privy Council which gives them priority access to certain types of information.
Local Sinn Féin MP Conor Murphy and party president Gerry Adams have repeatedly denied there was any republican involvement in Mr Quinn's murder.
According to Mr Murphy, the killing related to a criminal feud and IRA members "have no knowledge of this incident".
"I think there are a great many people who will try to make political capital out of an unfortunate, tragic and brutal murder, out of the grief of the Quinn family.
"There are people trying to make political capital out of that in a disgraceful way," he said.
Paul Quinn's father, Stephen Quinn, has asked Mr Murphy to produce information that his son was involved in a criminal feud or withdraw the allegation.
The Quinn family say he was killed because he beat up a local republican who had insulted his sister.
Two days after the fight Mr Quinn moved back to the family home and told his parents that he "needed to lie low".