On 30 January, 1972, a British paratroop regiment shot dead 14 Catholics at a civil rights march in Derry. Bloody Sunday remains a matter of controversy to this day.
Some 10,000 people gathered in Derry to march under the banner of the Northern Ireland Civil Rights Association against the policy of internment. The march had been banned. Within an hour of the march, 13 protesters were shot dead by members of the 1st Parachute regiment, the final victim died later.
The soldiers claimed they had been fired on by the IRA as they moved to make arrests. But the Catholic community maintains to this day that the crowd was peaceful and that the British Army murdered unarmed civilians.
Such was the scale of the outcry, the British government set up an inquiry under Lord Widgery. He concluded that: "At one end of the scale, some soldiers showed a high degree of responsibility; at the other end, firing bordered on recklessness".
The Londonderry coroner, Major Hubert O'Neill, did not share that view. At the inquests he said: "The army ran amok that day .. I say it without reservation - it was sheer unadulterated murder".
The British government later made out-of-court settlements with the bereaved families.
For three decades relatives of the victims and others who were present have led a campaign to find out what really happened on that day and to bring those they believed responsible to justice.
Bloody Sunday alone has come to be one of the most symbolic moments of The Troubles, a running sore that has never healed.
Irrespective of what actually happened on the day, it has come to be seen in the eyes of many sympathetic to the nationalist cause as representating the brutality of the British state against the Catholic community.
Others have long believed that peace and reconcilliation can only come to Northern Ireland when events such as Bloody Sunday are properly and honestly aired in the open as part of a process of healing.
The relatives' campaign came a step closer to completion when the British Government agreed to set up a new inquiry as part of its obligations under the Good Friday Agreement.
Click here for the BBC's full coverage of the new Bloody Sunday inquiry.