The relatives of four people murdered after alleged security force collusion with loyalist paramilitaries in the North are suing the police.
The families' solicitors issued a writ to PSNI chief constable Hugh Orde in Belfast yesterday.
A report last month by Police Ombudsman Nuala O'Loan found officers from the RUC Special Branch had protected their agent from prosecution for his part in 16 Ulster Volunteer Force murders in north Belfast between 1991 and 2003.
Andrea Murphy, deputy director at Relatives for Justice, is supporting their case.
She said: "They had no idea that the deaths of their loved ones could have been prevented and that there was such a high level of involvement in the killings. They want to bring responsibility to those who were responsible and could have prevented it and ensure that it doesn't happen again."
The families of Sharon McKenna, Peter McTasney, Gerard Brady and John Harbinson are taking the action.
They are claiming negligence, breach of statutory duty, conspiracy to injure, assault, trespass against the person and misuse of power in public office.
Catholic voluntary worker Peter McTasney (26) was murdered on February 24th, 1991. He was shot dead by the UVF in the living room of his home at Bawnmore Park, Newtownabbey.
The report said Mark Haddock, a leading loyalist paramilitary identified as Informant 1, was arrested and interviewed 19 times.
It said his Special Branch "handlers" conducted the main interviews and claimed notes were completed which did not reflect what happened in the interview. He was released without charge.
Sharon McKenna (27) was murdered on January 17th, 1993, while visiting a Protestant pensioner at his north Belfast home.
The dossier said a detective sergeant and a detective constable testified that Haddock had admitted to being one of the gunmen involved in the murder.
It also stated separate police documentation from the time records "high grade" information that Haddock was involved.
Catholic father-of-two Gerard Brady (22) was shot dead by the UVF on June 17th, 1994, after picking up two men in his taxi in Antrim.
The report said police had intelligence which linked Haddock and another police informant to the murder.
Ballistic tests have also connected the gun used in the incident to Haddock and other police informants.
John Harbinson (39) was murdered on May 18th, 1997, after being handcuffed and beaten.
The report said Special Branch had a significant amount of high-grade intelligence about the four main suspects for the murder, including Haddock, but did not pass it on to investigators.
It also said Special Branch withheld information that those who had carried out the murder had fled to a location in Ballyhalbert, Co Down where they were "safely ensconced".
Meanwhile, victims of loyalist collusion with the security forces are taking their campaign for justice to the United States during next month's St Patrick's Day celebrations.
Raymond McCord snr, whose RAF son Raymond (22) was battered to death and dumped in a quarry in November 1997, will be among a group in Washington DC to meet possible presidential candidate Hillary Clinton and other public representatives.
Mr McCord said: "I am going to present them with a report into collusion.
"Collusion affected the two communities, Protestants and Catholics, and as far as I am concerned it isn't a sectarian or political issue, it is a justice issue."
Mr Thompson and collusion victims Paul McIlwaine, Pauline Kennedy, Theresa Slane and Clare Reilly will also be present. - (PA)