How can I mourn a grandmother whom I have never met? How can I, a grizzled, grey-beard reporter, cast aside my professional response to an unspeakable atrocity and respond simply as a grieving grandson?

I am at a former fortress outside the Lithuanian city of Kaunas, staring into what used to be a giant burial pit, the site of one of the countless mass murders committed during the Nazis’ attempted extermination of Europe’s Jews.

My grandmother, Ilse Cohn, was among those whose lifeless bodies tumbled into that pit, one of 2,000 deported German and Austrian Jews who were shot there on a single day in November 1941.

I make a conscious effort to stop thinking like a reporter. I put down my camera and my notebook.

“Ilse,” I say quietly. “I was here. You are not forgotten.”

My mother was Ilse’s only child. She had fled from Germany just six weeks before the outbreak of the Second World War, leaving her mother behind after she had been refused a UK visa on the grounds that, at the age of 42, she was too old.

I have visited Kaunas twice over the past ten years: once on a reporting trip that I undertook with an American journalist friend, retracing the steps of our immigrant families, and then again with the film-maker David Wilkinson to appear in his remarkable film Getting Away

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Ilse Cohn

With Murder(s), which is to be shown next week for the first time on British television and on major streaming platforms in the US, Canada and Australia to mark Holocaust Memorial Day.

The film shines an unforgiving spotlight on the scandal of unprosecuted Nazi war criminals, the vast majority of whom were never brought to justice.

Among them was the man who ordered the murder of my grandmother, an SS colonel by the name of Karl Jäger, who returned to his hometown after the war and lived there undisturbed, under his own name, until he was finally arrested in 1959. He took his own life in a prison cell while awaiting trial.

All my mother ever knew of her own mother’s fate was what an aunt had written to her once the war was over. Two Gestapo officers had come to her flat early one morning: “Get dressed and come with us,” they said.

“Bring some warm clothes and enough food for four days.” They said she would be sent to Kovno (Kaunas) — and that was the last that anyone heard of her.

My mother never wanted to know more, but I did. The reporter in me wasn’t satisfied with only half the story, but I waited until after my mother died before I started trying to fill in the gaps.

Fortunately, the end of the Cold War had resulted in huge amounts of new Nazi-era information becoming available from what had until then been hidden behind the Iron Curtain.

It didn’t take long to establish that Ilse Cohn had been in the first batch of German Jews deported to Kaunas and that SS Colonel Jäger had compiled a meticulous report of the activities of his Einsatzgruppe, or mobile killing squad, detailing the 137,000 murders that he ordered following the Nazi takeover in Lithuania in June 1941. That’s how I know so precisely the date of my grandmother’s death.

“29.11.41: 693 Jewish men, 1,155 Jewish women, 152 Jewish children, evacuated from Vienna and Breslau.”

My mother knew none of this. All she knew — all she ever wanted to know — was that her mother had been deported and was never heard of again. For her, knowing more would have been unbearable.

But for me, at one generation removed, confronting the reality head-on — something that Europe’s post-war leaders so woefully failed to do — is the only chance we have to reduce the risk of it all happening again.

‘Getting Away with Murder(s)’ will be shown in two parts, on 27 and 28 January, on More4 and will be available after on All4.

Read more: Why did so many Nazis escape justice