The Saville Report into the events of Bloody Sunday is published on Tuesday. BBC Northern Ireland's Political Editor Mark Devenport reflects on the political legacy of the killings.
30 January 1972 may not have been the bloodiest day in the history of Northern Ireland's Troubles, but Bloody Sunday's significance in shaping the course of the conflict cannot be overstated.
Prior to Bloody Sunday some young Catholics, like Northern Ireland's Deputy First Minister Martin McGuinness, had already joined the IRA to fight against what they saw as an occupying British Army.
However, the actions of the Parachute Regiment in shooting dead 13 unarmed civil rights protestors immeasurably strengthened Irish republicans' arguments within their own community and provided the Provisional IRA with a flood of fresh recruits for its "long war".
In London, some government ministers may have approved the notion of "getting tough with the terrorists" who flouted their authority within so-called no-go areas like Derry's Bogside.
Yet the sheer number of lives lost on Bloody Sunday and the harsh international reaction to the killings convinced ministers and senior civil servants to re-examine their security policies and the kind of political advice they were getting from the unionist run government at Stormont.
Westminster decided that it must have full control over law and order. Stormont resisted.
So Bloody Sunday set in train the suspension of the Northern Ireland government in March 1972, which led to the decades of direct rule from London.
Despite several experiments at devolution, that era has only now drawn to a close with the restoration of policing and justice powers to Stormont after a gap of nearly 40 years.
If the Conservative government of Ted Heath believed that direct rule was the answer to their political dilemma, then they must have hoped that the judicial inquiry set up under the chairmanship of Lord Widgery would resolve the legal and ethical questions raised by the killings.
This proved a vain hope. When the tribunal submitted its report in April 1972 it was condemned by many in Derry as the "Widgery whitewash", an impression which only grew stronger over time.
The admission in 1992 by Conservative Prime Minister John Major that the victims were unarmed and should be regarded as innocent did not quell demands for a new independent inquiry, a request which was eventually granted by his successor Tony Blair.
When Lord Saville held his first hearing at Derry's Guildhall in April 1998, it was seen by many as an essential building block in any process of peace and reconciliation.
However, Bloody Sunday still has the potential to divide Northern Ireland's fractious politicians.
Few defend what the Parachute Regiment soldiers did that day. But many unionists questioned the concentration on one incident at the start of what turned out to be the Troubles' bloodiest year.
Nationalists counter by arguing that state violence, apparently covered up at the highest levels, raised different and more searching questions than the paramilitary killings on either side.
Unionists say once Sinn Fein's Martin McGuinness admitted being the IRA's second-in-command in Derry on the day of Bloody Sunday, he should have also come clean about the many IRA murders carried out in the same area that year.
On this level, the Bloody Sunday Inquiry, set alongside moves like the early release of paramilitary prisoners, fed a growing sense of alienation among unionists regarding the peace process after the signing of the Good Friday agreement.
This was compounded by growing concern over the cost of the Inquiry. As the legal bills ran into tens of millions then hundreds of millions of pounds not just unionists, but Conservative and Labour MPs questioned whether this was an appropriate use of resources.
Nationalists responded by pointing out that much of the expense had been generated not by lawyers acting on behalf of the victims families, but those representing soldiers and the Ministry of Defence.
In March 2010 the politician who is now Northern Ireland's Justice Minister discovered first hand the continuing sensitivities surrounding the Bloody Sunday inquiry.
In a leaked e-mail, the Alliance leader David Ford, called the Saville inquiry "pointless". Some unionists commended him, but nationalists expressed outrage.
In what was widely interpreted as a necessary move before he could take on the Stormont Justice portfolio, Mr Ford went to see the Bloody Sunday families to apologise in person.
The creation of the Saville Tribunal was meant to help heal the wounds left by Bloody Sunday.
But such are the dynamics of Northern Ireland that tending to one group of victims only serves to stir painful emotions amongst others.