By Kevin Connolly
Ireland correspondent, BBC News
The unit is looking at hundreds of cases from 40 years of conflict
It was a thought that struck me often in the months in which I prepared a pair of Radio 4 documentaries - History's Witness - to be broadcast this month.
The problem is how to address the past. Public enquiries are costly and slow. Lord Justice Saville's investigations into the shooting of demonstrators by British paratroopers in Derry in 1972 has already taken nine years, and cost £180m.
That is simply to uncover the truth about 14 deaths, out of almost 4,000 on a single day, out of nearly 40 years of conflict.
A truth commission based on the South African model would founder on the most basic of questions. For example, is Sinn Fein the political wing of the IRA?
Republicans say it is not.
Every academic, journalist, police officer and member of rival political party in Ireland would says that it is.
Even on the most basic issues, the common language needed to create a body with wide-ranging powers to adjudicate on the agonies of the past simply is not there.
It was perhaps that kind of reasoning that led to the creation of the Historical Enquiries Team (HET), a unique experiment in policing in which teams of retired detectives have been assembled to re-open the files on every killing of the Troubles.
When the team was launched, there was bold talk that its work might lead to prosecutions in cases where new evidence could be uncovered, or where new forensic techniques could be applied to old evidence.
That aspect of the team's work never seemed likely to succeed.
Inquiries are costly and take a long time
It is one of the conditions of the Good Friday Agreement that anyone convicted of a murder committed during the troubles will serve no more than two years in jail, even in the highly unlikely event that a prosecution can be brought.
Even where physical evidence can be found, it is unlikely to have been stored in 1972 with the degree of care needed in forensic science techniques that were not discovered until 30 years later.
The team's strength, though, lies in its ability to help individual families find out more about what happened to a son or a mother or a husband or a father who is among the dead.
Often they have been told very little and their questions are heart-breakingly simple: "did my son suffer at the end?" or "did my husband say anything in his last moments?"
Policing culture - not just in Northern Ireland, but across the developed world in the 1970s - tended not to put families at the centre of investigations.
There were, back then, no officers specially trained to counsel families and to explain to them the often puzzling workings of the criminal justice system.
I have been following the work of the HET as they have dealt with four cold cases.
But do not think of them in that way. Instead, think of them as the stories of four families in whose hearts a clock stopped on the day bereavement's cold hands touched them.
Episode One of History's Witness, written and presented by Kevin Connolly and produced by Rachel Hooper, is broadcast on Radio 4 at 1100BST on Monday 2 July.