Justice must be served for nine people killed in NI bombings who were "failed" by a cover-up of a priest's suspected role, victims' relatives have said.
The 1972 bombings in Claudy were among the worst atrocities of the Troubles.
The Police Ombudsman found talks between the Catholic Church, the police and the government led to a priest suspected of involvement in the attack being moved to the Irish Republic.
No action was ever taken against Father James Chesney, who died in 1980.
Nine people were killed after three no-warning bombs exploded in the small County Londonderry village on 31 July 1972.
No paramilitary group has ever claimed responsibility for the attack, and no-one has been convicted of it.
Police Ombudsman Al Hutchinson's report confirmed that detectives in 1972 believed that Father Chesney was director of operations for the south Derry IRA and was involved in the planning of the Claudy attack.
However, after discussions between the police, the Northern Ireland Office and the Catholic Church it was decided not to pursue Fr Chesney and he was moved to a parish across the border in County Donegal.
'Hard to take'
Mark Eakin, whose eight-year-old sister Kathryn was killed in the blast, said he would like to see someone brought before the courts.
"I would like to ask the British government if they would now step in and investigate this thing further, give the PSNI of today, who are still trying to investigate, more resources," he said.
James Miller, who was two-years-old when his grandfather David was killed in the third bomb, called on the British government and prime minister to disclose more information.
"To be told that after the bomb, he was moved to Donegal and continued his activities - it's very hard to take on board," he said.
Northern Ireland Secretary Owen Paterson said the government was "profoundly sorry" that Fr Chesney had not been properly investigated.
The head of the Catholic Church in Ireland, Cardinal Sean Brady, said the church was not involved in a cover-up over the role of Fr Chesney.
Sinn Fein, the political party closely indentified with the IRA, said the deaths in Claudy were "wrong and should not have happened." The party repeated its call for an independent international truth commission.
The Police Ombudsman's office, which holds the police to account, began its probe into the original investigation eight years ago.
Al Hutchinson's report, published on Tuesday, found that by acquiescing to a deal between the government and the Catholic Church to move Fr Chesney to a parish in the Irish Republic, the Royal Ulster Constabulary was guilty of a "collusive act".
He said this had compromised the investigation and the decision "failed those who were murdered, injured or bereaved" in the bombing.
He said that if officers involved were still alive, "their actions would have demanded explanation, which would have been the subject of further investigation".
As well as investigating complaints made against the Police Service of Northern Ireland, the Police Ombudsman also has the authority to look at investigations carried out by their predecessors, the RUC.
Mr Hutchinson said some detectives' attempts to pursue Fr Chesney were frustrated ahead of a meeting between Northern Ireland Secretary William Whitelaw and the leader of Ireland's Catholics, Cardinal Conway.
There, it was agreed that the priest would be moved to a parish in Donegal, just over the border in the Irish Republic.
The Ombudsman found that the chief constable, Sir Graham Shillington, was made aware of this decision.
Mr Shillington said he would "prefer a move to Tipperary", about 200 miles from the border.
Fr Chesney, who denied involvement in terrorist activities to his superiors, was never arrested.
Cardinal Sean Brady said on Tuesday: "The Church was approached by the secretary of state at the instigation of senior members of the RUC," he said.
"Furthermore, the Church subsequently reported back to the secretary of state the outcome of its questioning of Fr Chesney into his alleged activities.
"The actions of Cardinal Conway or any other Church authority did not prevent the possibility of future arrest and questioning of Fr Chesney."
Both Protestants and Catholics were killed in the blasts.
The youngest victim was eight-year-old Kathryn Eakin who was cleaning the windows of her family's grocery store when the first bomb exploded.
The other people killed were Joseph McCloskey, 39; David Miller, 60; James McClelland, 65; William Temple, 16; Elizabeth McElhinney, 59; Rose McLaughlin, 51; Patrick Connolly, 15; and 38-year-old Arthur Hone.
Mr Hutchinson said that he accepted some of the decisions taken "must be considered in the context of the time" but added that the conspiracy still amounted to collusion.
"I accept that 1972 was one of the worst years of the Troubles and that the arrest of a priest might well have aggravated the security situation.
"Equally I consider that the police failure to investigate someone they suspected of involvement in acts of terrorism could, in itself, have had serious consequences."
He said he had found no evidence of criminal intent by anyone in the government or the Catholic Church.
The Police Service of Northern Ireland, which replaced the RUC in 2001 following a reform of policing, said the investigation into the Claudy bomb was now under the remit of the Historical Enquiries Team, which investigates unsolved murders from during the Troubles.