IT took many years for my grandfather Karl's story to come out, that he had been a paid-up member of the Nazi Party, an officer in the SS, and a manager of slave labour in concentration camps such as Auschwitz, Buchenwald, Dachau, Sachsenhausen and many others.

For most of his life, my dad, who had grown up in Germany before, like his sister Anne, emigrating to Scotland, had not told his family anything about his early childhood in Germany.

Once, he let slip that Karl had been a "pen-pusher" in Dachau. It wasn't until I was 50 when, searching the internet, I came across my grandfather, and references to what he had done: ''crimes against humanity'', ''use of slave labour'.

He was a middle-ranking functionary in the SS. My dad had called him a pen-pusher, which made me assume he had been a clerk in a wee office, filling out forms, but he was more than that. He ensured that thousands of concentration camp inmates were pushed into work. His work meant that he was a regular visitor to such places, and he would surely have been a witness to scenes of unimaginable brutality.

I think Karl had a good idea about what was going on. He knew, for example, how many Jews were killed on a certain date in November 1942; that sort of information was not known by the general populace but it was known by him, and it must been known by everyone who was working for the SS at that time.

The day after Germany surrendered, Karl was driven away in a US army jeep. He was interned for three years in former PoW camps and, in July 1948, he appeared before a denazification tribunal.

Afterwards, he was a broken man. It's impossible to say for sure, but I don't think he was broken by what he had done. It was more by the fact that everything had gone wrong for him. Germany had lost the war. He had lost his son, Dieter, in the battle for Würzburg in April, 1945. He had lost his job, he no longer had a status. Two of his children had left for Scotland. I'm guessing that these would have been the principal factors. I feel, too, that maybe he distanced himself. He said, 'I didn't kill anyone: I was just doing my job'. But my dad definitely said that when he came back home, he was not the same person.

I've thought a lot about his wife, Minna, since writing my book about Karl. She wouldn't have wanted to have voiced any criticism of the regime, even to her own children; there was always the risk that little children would go into school and say, 'My mum says this...', and then they would all be straight into the concentration camp.

I was massively conflicted while researching and writing the book. It was a big trial. Psychologically, it was difficult for me to start. My immediate family were not keen on me doing it. One worry was what the book would do to our relations with my uncle Eki in Germany, but he has been fantastic and very open, saying, 'Go ahead, tell the story'. In the book I've tried to be as honest and open as possible.

Germany has long been reunified. I suspect Karl would have approved. Fundamentally, he prized loyalty. He was a man of complicated motivations, He was a strict father but he could be relaxed and garrulous; and it emerged that during the war he arranged deals with the Gestapo to have some inmates freed. Some of them spoke in in his favour at his tribunal.

* A Nazi in the Family (Short Books, £14.99). Derek Niemann's website is www.whispersfromthewild.co.uk