Families of eight men killed by a UVF gang that included members of the RUC won a victory in the European Court of Human Rights yesterday.
The Court ruled that their human rights had been violated because a 1999 investigation into new allegations about the 1976 killings was not sufficiently independent.
The families were each awarded 5,000 euros and legal costs.
The Government said it was "disappointed" with the decision, but pointed out that the court found no problem with the later investigations involving the PSNI and Historical Enquiries Team.
The 1976 attacks included the sectarian murders of two sets of Catholic brothers, the Reaveys and O'Dowds, and attacks on two bars in South Armagh.
John, Brian and Anthony Reavey were shot at their home in Whitecross, Co Armagh, on January 4, 1976. Joseph, Barry and Declan O'Dowd were murdered near Gilford the same night.
Their killings came during a furious bout of sectarian violence in the area that led to the Kingsmills massacre the next evening.
Cases were also brought by the families of Colm McCartney, who was murdered at Altnamackin in August 1975; Trevor Brecknell, who was murdered at Donnelly's Bar, Silverbridge in December 1975; and by Michael McGrath, who was wounded in a gun attack at the Rock Bar, Keady in June 1976.
RUC members, including an officer who was on duty at the time, were later jailed for the Rock Bar attack. Guns used in that attack had also been used in some of the killings.
The killings took place before the UK adopted the Human Rights Act, and would normally be outside the jurisdiction of the European Court.
But the cases were based on a new investigation launched in 1999, after former RUC officer John Weir - once jailed for a loyalist murder - alleged that police officers and at least one UDR member were part of the killer gang.
Yesterday the Court ruled that "the investigative response to Weir's allegations lacked the requisite independence in its early stages".
"There has been, in that respect alone, a violation of Article 2" of the European Convention on Human Rights.
The families' solicitor, Fearghal Shiels of Madden and Finucane, said: " In 1999, the RUC purported to conduct a police investigation into John Weir's allegations.
"The RUC took no steps to interview John Weir, and irrespective of the cogent and credible evidence of widespread collusion by members of UDR and RUC with a loyalist murder gang based in Mid-Ulster, concluded that his allegations were false.
"The findings by the European Court of Human Rights that the families' human rights were breached by the UK Government vindicates the families' central contention that there was a total lack of independence, transparency and accountability on the part of the RUC, in investigating the activities of this murder gang."
Mr Shiels indicated that the families will meet their solicitors "to discuss the important implications of the ruling".
A spokesman for the Northern Ireland Office said: "While the NIO is disappointed by the judgment, it is important to note that the court found a lack of independence only in the initial RUC investigations into John Weir's allegations and not the subsequent police investigations.
"The court also rightly recognised that the PSNI is an institutionally distinct organisation from the former RUC."