Gitta Sereny: Writer best known for her books on Mary Bell and Albert Speer

Gitta Sereny was a biographer and journalist whose work involved researching and writing about some of the most notorious individuals of the 20th century, including murderers and war criminals. She is best known for her book Cries Unheard: The Story of Mary Bell (1998), which documents the life of the child who was involved in the killing of two younger children, as well as for her public battles with the revisionist historian David Irving and her biography of Albert Speer.

Sereny was born in 1921 in Austria, the daughter of Ferdinand Sereny, a Hungarian aristocrat and the German former actress Margit Herzfeld. Aged only 11 she read Hitler's Mein Kampf and four years later heard him address crowds in Vienna. It was while visiting the Nuremburg war crime trials in 1945 that she first saw Albert Speer, the architect and friend of Adolf Hitler who was to become the subject of one of her biographies.

In 1948 she married Don Honeyman, a photographer best known for his famous black-on-red portrait of Che Guevara, and they remained together for the next 63 years.

In Sereny's first book, The Case of Mary Bell: A Portrait of a Child Who Murdered (1972), the eponymous subject was aged 11 when she was convicted of the manslaughter of Martin Brown, aged four, and Brian Howe, aged three, in Newcastle in 1968. Sereny had first encountered the case while reporting on it for the Daily Telegraph. For this work she interviewed family and friends but did not meet with the subject herself until much later. In a preface to the 1995 edition of the book, published following the murder of James Bulger, she poses the questions "How, first of all, can it be right to subject young children to the awesome formality of a jury trial? How can a jury be expected to understand the thought processes, the emotions or language of children?"

For Into That Darkness: from Mercy Killing to Mass Murder, a study of Franz Stangl, the commandant of Treblinka (1974) Sereny went a step further, making extensive use of 60 hours of face-to-face interviews with the subject of the book. Stangl had been tracked by Simon Wiesenthal to Brazil in 1967 and was arrested there. He died in prison in Düsseldorf in 1971, one day after the last interview with Sereny, in which he finally admitted his guilt. Asked about her choice of subject, she said later: "Because he had a world of experiences which he had never thought of analysing and to be confronted with someone who analysed what he said and played it back to him... it became very important to him and to me."

Sereny tackled David Irving's controversial book Hitler's War (1977) in its year of publication. In this work Irving claimed that the Nazi leader had not known about the extermination of the Jews until October 1943. She recalled: "I was intrigued by his book... My first thought was that it was very good. I thought his idea – which turned out to be wrong, namely that Hitler did not know about the Holocaust – was just possible."

Reviewing the book in the Sunday Times she put forward the view that it was "closer to theology or mythology" rather than a work of history. When she and Lewis Chester looked more closely at the evidence they found a remark about the Holocaust from Otto Günsche, a primary source used selectively by Irving, who had said "one must assume that he [Hitler] did know". Following the publication of Sereny's April 1996 article in The Observer titled "Spin Time for Hitler", Irving decided to sue, claiming that the paper failed to publish his reply to the article and alleging that Sereny had pursued a campaign of defamation against him.

Covering the lawsuit in this newspaper, Jack Sullivan interviewed Sereny, who said of Irving: "He is obsessed with a number of things, his feelings for Hitler and his own feelings about the Jews. He has great sympathy with Hitler and that makes him dangerous. I am the red cloth for him because I am as familiar as he is with the areas he writes about."

Sullivan summarised the situation thus: "Whatever the outcome, thecareers of Irving and Sereny seemdestined to remain interlocked."Irving decided to drop the suit but lost another case against Penguin books and Deborah Lipstadt, which drove him to bankruptcy.

In 1995 she published Albert Speer: His Battle with Truth, in which she reveals that Speer knew about the plan to exterminate the Jews but that his adoration for Hitler prevented him from disclosing it. She admits in the book how she "grew to like" the subject and his wife over the course of interviews at their retirement home. John Elson, reviewing the book in Time, said "Sereny has probably captured Speer's aloof, elusive persona as well as any writer could. She also usefully reminds that Hitler, for all the evil he inflicted, was not a cartoon monster but a man with immense charisma and even some charm." The book became the subject of the play Albert Speer (2000) by David Edgar, directed by Trevor Nunn.

That same year she reprised the subject of Mary Bell, controversially paying her for her time. The result was Cries Unheard: The Story of Mary Bell (1998). In a letter to the parents of one of the victims she clarified that the purpose of the book "was not to re-live these terrible crimes, but to find some understanding of how they could happen". Explaining her reasoning for paying Bell, she said, "If I hadn't done so, I would have made myself guilty of doing what has been done to her virtually since she was born: to USE her..."

Gitta Sereny, author: born Vienna 13 March 1921; married 1948 Don Honeyman (died 2011; one son, one daughter); died 14 June 2012.