In the 1990s a relative of mine was told by a senior police officer that there were very few murders in Northern Ireland that had entirely confounded detectives.
There had been many, many murders that had never led to a conviction (more than 1,000 from the Troubles, for example), but in most of them police had been able to attribute the killing to an individual or group of individuals, even if this had not been provable in court.
But there was one killing about which they had no such instincts or leads: that of Inga Maria Hauser, the 18-year-old German backpacker found in Ballypatrick Forest, north Antrim in 1988, weeks after she stepped off a boat from Scotland.
It was a murder that has been an unresolved stain on the record and reputation of the Province ever since.
There is something particularly poignant about the fact that she had the confidence to travel here on her own, early in her adulthood, and yet was to die in such a fashion.
We can only hope that she had no warning of her impending death and did not have time to suffer.
Her last diary entry makes for almost unbearable reading, given that we all know the way in which her visit to Northern Ireland would turn out:
“Went from Glasgow to Ayr and from there to Stranraer to get over to Ireland.
“Saw the sea. Beautiful and mysterious. Wonder where I stay tonight. Need more money.”
Like most people in Northern Ireland who was at or near adulthood when Inga Maria went missing, I remember well the case. She was only a couple of years older than me.
Her disappearance featured on the BBC TV show Crimewatch, which then had a vast UK wide audience, raising the hope that perhaps there would be a breakthrough in the investigation. But no crucial breakthrough ever came.
However, if it is true that police were initially entirely stumped by the case, then at some stage over the decades they seem to have come to have a reasonable understanding of what happened.
Or so they have been implying in recent years, on the intermittent appeals for information that detectives issue as the anniversary of Inga Maria’s murder keeps coming round.
This year is not the first time that they have hinted at an almost Shakespearean element to the tragedy of this young woman’s death: that someone with information about the murder is finding it very hard to live with.
I wonder how they know this.
Has someone sent an anonymous note?
Has someone confessed facts relating to the killing to a third party, perhaps even in the knowledge that some of it might have to be passed on to police?
Is the person who feels guilt the wife or mother of the murderer?
Is it a friend of the killer who was called to the scene in a panic?
I have not talked to anyone with information about the investigation so I am merely speculating on possible scenarios.
Even if the culprits or accomplices were aged only in their 20s at the time of the murder, they would now be in their 50s. But they might be much older than that and conscious of their own impending death.
Someone, somewhere might be living in a daily hell with regard to this murder, tortured by their memories.
Perhaps the killer is in that very state of hell.
Or perhaps the killer is incapable of any empathy or regret or remorse, a different sort of hell.
Or perhaps the killer is dead.
Few people in Northern Ireland who remember this dreadful murder will ever have a sense of resolution about the case until the facts are established about what happened to the vulnerable, trusting young woman who came to our shores at a time, as Superintendent Raymond Murray puts it, “when a lot of people wouldn’t come”.
But it is her family in Germany, including a sister, who have endured what must have been a 30-year nightmare.
Her father Josef is now dead and her mother Almut is said to be ill.
The fate of Inga Maria is a bleak story of so far unrelenting horror and mystery, but it might yet become one of horror and partial redemption if the person who is said to be struggling with their conscience was to come forward and explain what happened.
• Ben Lowry (@BenLowry2) is News Letter deputy editor