As a Jew in Nazi Germany, Marianne Ellenbogen spent the second world war on the run. Biographer Mark Roseman was amazed by her recollections, especially when some of them proved to be wrong

I began writing Marianne Ellenbogen's biography almost by accident. We had met briefly seven years earlier in 1989 when, alerted by a German museum, I conducted a short interview with the unknown Liverpool housewife and gained a first glimpse of her astonishing wartime survival story. In 1996, when Marianne was widowed and very ill, she invited me back to Liverpool to see if a few of her papers were archive-worthy. I went as a courtesy, but at lunch her son - more aware than I how little time was left - asked me to record his mother's experience of the Holocaust. Marianne did not demur when I suggested I should return with a tape recorder. Thus the book was born.

Because I plunged into the project with so little preparation, I did not reflect much on methodological issues, such as the question of memory and accuracy. The things Marianne told me were in any case so remarkable, and she was such a persuasive witness, that it did not occur to me to challenge them.

Her family had been patriotic, well-established Jews, who had not resolved to leave Nazi Germany until it was too late. With other educational opportunities closed, the young Marianne - she was born in 1923 - left her parents behind in Essen and blossomed in Berlin, training as a kindergarten teacher. In 1941, she was called home when the family were selected for deportation to Lodz. At the last minute, they were pulled back from the crowd at the collection point in Essen and sent home. Marianne told me that it was the Abwehr, the German Wehrmacht's intelligence arm, that had protected the family, though she did not know why.

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In 1943 after two agonising years, the protection collapsed. The Gestapo came to the house to collar them for deportation to Theresienstadt. Marianne escaped, seeking refuge with a little-known leftwing group, the League for Socialist Life, and for two amazing years she criss-crossed Nazi Germany without papers, staying with one group member after another until her liberation in Dusseldorf in 1945. During this time, her parents, brother and extended family were murdered, most of them in Auschwitz.

After three lengthy conversations, Marianne asked to pause while she recuperated. She was showing signs of recovery when, unexpectedly, she died. Alongside personal feelings of loss, I was suddenly very conscious of the gaps in her testimony. Moreover, detached from Marianne's compelling presence, some of the things she had told me seemed rather unlikely. For example, she claimed that while listening illegally to the BBC in 1944 she heard that a particular transport of people from Theresienstadt to Auschwitz was gassed on a particular date and that she knew her family was on that transport. On reflection it sounded highly implausible.

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Over the following months, an extraordinary range of sources began to emerge. In Germany, the Gestapo records on the family survive and are among the most informative that exist about the shadowy role of the Abwehr. It then transpired that the leader of the League for Socialist Life had kept a detailed diary. Back in Liverpool, trying to sort out his mother's effects, Marianne's son uncovered folder after folder of forgotten and hidden papers, including Marianne's own diaries, one written while she was on the run. Marianne's correspondence and other sources led me down a lengthening chain of school contemporaries, friends from her youth group, fellow students from Berlin, business associates of her parents and members of the underground, now dispersed across Britain, Germany, France, Sweden, Israel, the United States and Argentina. These sources confirmed, without exception, all Marianne's most dramatic assertions. She really had known which transport had taken her parents to Auschwitz. And, while on the run in June 1944, it really had been her terrible lot to hear on the BBC a precise report of their fate. Indeed, taken as a whole, Marianne's testimony and the papers in her possession transformed my sense of what it was possible to communicate and know in Hitler's Germany.

But on other, subtler matters, difficulties began to emerge. I met a girl whom Marianne remembered as one of her biggest tormentors at school. When I asked this Frau Sparrer what relations between Jewish and non-Jewish pupils had been like, she responded, "A-1 - excellent!" I revealed that Marianne had remembered her as an anti-Semitic bully. "Me?" she asked, clearly shocked and distraught. From her manner, I felt sure she was suppressing memories, probably even to herself. But my black-and-white picture began to dissolve as one after another of Marianne's non-Jewish classmates, many of them far more sensitive and self-aware than Frau Sparrer, recalled the school as a relatively tolerant zone. Even one of Marianne's Jewish friends wrote that she had no memory of persecution within the school gates.

Hard though it is to distinguish between contemporary observation and the effects of memory, it seemed that the Jewish experience at Essen secondary schools in the 1930s was far from uniform and that the diversity in testimony reflected authentic differences in perceptions at the time. But in other matters, there could be no question of different viewpoints coexisting. For example, Marianne had a clear memory of her fiance's last night before his deportation. She told me she had accompanied him and his family to a barracks where they were being held prior to deportation and had risked spending the night with him, all the while pleading with him to make a run for it. A complex series of clues established that her fiance's last night had been spent neither in the barracks nor with Marianne: indeed, he had written a moving letter to her that she received only later. It was, as it turned out, a friend of Marianne's who had spent the night with her own partner and Marianne had later come to believe the story was her own. Marianne also had a powerful and clear memory of her flight from the house watched over by the Gestapo. Her memory was contradicted by the Gestapo report of the escape; not a very credible source, we might think, but one independently corroborated by someone who had spoken to Marianne immediately after the war. Restitution papers show that soon after that, details in Marianne's account of this escape were already shifting. Such changes were particularly noteworthy because when I spoke to Marianne the memory of these partings and departures seemed so vivid and traumatic.

Of all the documents, Marianne's wartime diary was the most dramatic. But it did not correspond at all to what I had expected. I had assumed that on the run Marianne would see herself primarily as a Jew in hiding and as a victim. But the diary revealed that at the time Marianne had not in any way seen herself or her problems in the light of Jewish persecution. Instead, she clearly saw herself as part of an idealistic political grouping, seeking to lay the foundations for a better society. It seemed to me that in later life Marianne had lost sight of the person she had once been.

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At a number of recent conferences on the Holocaust, survivors have been understandably upset at what they see as historians' over-readiness to question and re-examine their testimony. But the discrepancies I encountered left me with no choice. Not only that, but in trying to understand the forces that influenced Marianne's memory, it did not seem to me I was denying the authenticity of her account. On the contrary, I was seeking to understand who she was and what she had gone through. For example, a pattern emerged in the way Marianne's psyche changed the memory of her traumatic partings and leave-takings (from her fiance, from her parents and family). This pattern seemed clear evidence that Marianne was unconsciously seeking to master, and come to terms with, an unmasterable past. The very discrepancies in Marianne's testimony, in other words, came to seem the most poignant and powerful evidence of the traumatic experiences that lay behind her.

Mark Roseman is professor of history, University of Southampton. His book A Past in Hiding is published next week by Allen Lane, The Penguin Press, Pounds 18.99.