Sinn Fein's Caitriona Ruane told MLAs that British special agents were "involved at every stage" of the 1994 Loughinisland killings, on 27 June 2010.
Six men, all of them Catholics, were murdered by the UVF in the Heights Bar in the village.
A recent report by the police ombudsman, Al Hutchinson, found that there were failings in the investigation. However there was insufficient evidence of collusion.
Ms Ruane said the victims' relatives had not got justice. "They have been failed by the RUC, by the PSNI and now by the police ombudsman," she added.
Jim Wells of the DUP said totally innocent people had been gunned down at Loughinisland.
He said Sinn Fein could not "have it both ways" and called on them to co-operate with the Historical Enquiries Team (HET).
John McCallister of the UUP said the Loughinisland killings had come at a "truly dreadful time for Northern Ireland".
He said you could not have "a situation where if you don't like the report you rubbish the report, and you rubbish the office."
SDLP leader Margaret Ritchie remembered the Loughinisland killings as "the brutal murder of six of my neighbours".
She said there was undoubtedly collusion in this case and called on the ombudsman to resign.
"I'm no longer certain that the office is independent," she added.
The UUP's Danny Kennedy brought a second matter of the day to the assembly.
Ten Protestant workmen were murdered in the Kingsmills massacre of January 1976.
Mr Kennedy said that the killings had a profound personal impact on him as he had known nine of the victims personally.
He said the leadership of the republican movement had "serious questions to answer" and called on them to give information to the police.
William Irwin of the DUP described the killings as "a squalid sectarian act by a group of criminals" and noted that the HET had said the murders were carried out by the Provisional IRA.
Sinn Fein's Conor Murphy said "what happened there was wrong. It shouldn't have happened." He extended his sympathy to the relatives of the men and called for the establishment of an international truth commission.
Dominic Bradley of the SDLP said what happened at Kingsmills was "one of the worst incidents of murderous slaughter to take place here."