Father James Chesney was the 'prime suspect' after nine people, including an eight-year-old girl, were killed and 30 injured when three car bombs exploded in the quiet Northern Ireland village of Claudy in July 1972.
The police chief wrote a letter which began an official cover-up, enabling the suspected terrorist to evade justice and move across the border to the Irish Republic, where he died from cancer eight years later at the age of 46.
The protection given to Chesney echoes action by the Catholic Church in Ireland to shield its priests from allegations of child sex abuse. Scandals surrounding the abuse and subsequent cover-ups have helped topple the Church from its once dominant position in Irish life.
This information was believed to have been shared with Cardinal William Conway, the head of the Catholic Church in Ireland, and the two men held secret talks in December to discuss Chesney.
A letter about the discussions sent to police stated:'The Cardinal said that he knew that the priest was a very bad man and would see what could be done.' The priest was said to have 'strenuously denied' any involvement in the bombs when quizzed by a church 'superior'.
It was later decided to move Chesney to a parish in County Donegal, despite objections from the Chief Constable Sir Graham Shillington, who preferred sending him to Tipperary, a safer 200 miles from the border.
Indeed, Chesney was known to have regularly crossed back into Northern Ireland and police intelligence suggested he continued to be involved with the IRA, but he was never arrested or questioned.
No one was ever charged with the Claudy murders.