Killings that may have involved state collusion should be probed by public inquiry, and not a truth commission, campaigners have claimed.
This came after Chief Constable Hugh Orde yesterday repeated his belief that police resources cannot cope with investigating "historic" cases from the troubles.
He has pointed to the limited prospect of securing convictions in such circumstances and has argued in favour of a type of 'truth commission' for Northern Ireland.
The British and Irish governments, however, have identified six cases where it is alleged security forces in Northern Ireland or the Republic may have colluded in murder.
Last night (Friday) the SDLP backed the truth commission proposal, but said that the six cases 'ring-fenced' by the two governments should still be dealt with separately.
The party's deputy leader Brid Rodgers was speaking on the eve of today's anniversary of the murder of solicitor Rosemary Nelson.
The solicitor was killed in a loyalist car bomb attack on March 15 1999. She had complained of security force intimidation, and the allegations of collusion in her case continue to linger.
Retired Canadian Supreme Court judge Peter Cory has been asked to report on six cases identified by the governments.
These include Mrs Nelson's, that of fellow solicitor Pat Finucane, LVF leader Billy Wright, sectarian murder victim Robert Hamill, Lord Justice and Lady Gibson, and RUC officers Bob Buchanan and Harry Breen.
Last night the SDLP, Sinn Féin and human rights groups, marked the anniversary of Mrs Nelson's murder by repeating their support for an inquiry.
The Irish News this week reported that a former soldier and a police informant are among 10 suspects identified by investigators in the case, though no-one has ever been charged over the murder.
Investigators have said they have found no evidence of collusion in the case.
The senior English officer leading the investigation stepped down late last year and a successor has yet to be appointed.
Ms Rodgers said that the murder investigation had been "wholly unsuccessful".
"The concerns over the allegations of security force collusion in this case remain, and the investigation has been unable to establish the truth of what happened," she said.
She said her party believed that the six cases presented to judge Cory were "now being dealt with separately", but backed the need for a truth comm-ission to help bring closure for thousands of unsolved murders.
Sinn Féin's Dara O'Hagan said: "The investigation has determined that known loyalists who are suspected of involvement in Rosemary Nelson's murder were also acting as Special Branch informers.
"One of the suspects is a former member of the RIR.
"Once again serious concerns are raised about the role of the intelligence services in the murder of a human rights lawyer," the Sinn Féin councillor said.
"The current investigation is incapable of finding the truth behind the murder of Rosemary Nelson."
The Northern Ireland Human Rights Commission last night said of Mrs Nelson's killing: "The commission regrets very much that the extensive police inquires conducted so far have not yet resulted in any person being charged with that murder.
"In the commission's view this makes a public inquiry into the killing all the more necessary and urgent," the commission said.
