A year or so ago, literary journalists were talking in hushed tones about "the secret" of Gitta Sereny's next book. The paperback of her acclaimed biography of Hitler's technocrat, Albert Speer: His Battle With Truth, had just been published, and her star had never been higher.

But what was this "Project X", as the publishers Macmillan were calling it? Some of the suggestions were outlandish: one was that Sereny had found Martin Bormann in the South American jungle.

Only a few people at Macmillan knew the truth - that the distinguished investigative journalist and author was going back over old ground. In 1972, Sereny published The Case Of Mary Bell, which told the story of the conviction in 1968 of the 11-year-old girl from Tyneside, for the murder of two boys. Martin Brown, four, was strangled and Brian Howe, three, asphyxiated.

In the new book, Cries Unheard (about which she is contractually barred from talking), Sereny attempts to go beyond the brute facts of the Mary Bell case, to understand the psychological trauma that drove the daughter of a Newcastle prostitute and alcoholic petty criminal to murder.

Put like that, it seems a noble ambition, representing just one element in a fiercely pursued wider project of Sereny's - to locate the origins of evil in society and particular individual histories, rather than (as happened when the Bell case first came under scrutiny and, more recently, with the 1993 murder by two 10-year-olds of two-year-old Jamie Bulger) in a medieval notion of original sin.

In the past, Sereny has employed the rhetoric of horror films to highlight the preposterousness of such a notion: "Are we still not beyond the point where we call sick children monsters and believe in evil seeds and evil birth?" she asked.

But the fact that Bell - released in 1980 and now living under an assumed name in the north of England - was paid about £50,000 for her collaboration in the new book has caused some controversy, to the extent that the British Home Secretary is reputedly considering bringing such payments into the remit of the Proceeds of Crime Act.

The controversy has also focused the spotlight on Sereny. What has hitherto been seen as the heroic pertinacity of a journalist and author who has spent much of her life uncovering the facts - the psychological "facts" as well as the historical ones - about individuals associated with the Holocaust, is now in danger of being presented as ghoulishness and opportunism.

In person, Gitta Sereny is small, round and physically self-effacing. Only in conversation does she seem forceful and penetrating. Her face is rather stern, even forbidding, in repose, but can break out into a broad, open smile.

Dressed in elegant black at the Garrick Club launch party in London of the Speer book, she tended to marginalise herself, moving to the side of the room as if she were an observer of her own event - until one crucial moment, when she made every one sit down on the floor for her speech.

Some saw it as a significant testament to a certain ruthlessness and manipulativeness. "You should never underestimate Gitta's hold over your feelings of guilt," as one of her editors has said.

Sereny has long been used to accusations of exploitation. Given her subject matter, this is not surprising. As well as biographies of Speer and Franz Stangl, commandant of the Treblinka death camp, she has written a 20,000-word investigation of the Bulger case, numerous in-depth murder investigations for quality broadsheets and magazines, and analyses of contemporary neo-Nazism.

Sereny was born in Vienna in the 1920s: the date seems to be kept deliberately vague in publicity material, but by most accounts she is 74. Her family were Anglophile, Hungarian landowners - not Jewish. Owing to her father's love of the English, she attended a boarding-school in Kent during her early youth. It was there, extraordinarily, that she read Mein Kampf.

By the age of 14, she was back in Vienna permanently, studying to be an actress and, inter alia, hearing Hitler make speeches to adoring crowds in the mid-1930s. By then, as she has written, "I had become terribly, achingly aware of wrong, wrong in my small world and in the world beyond it." She saw the disappearance of Jews gather pace and knew then that she too - for her own reasons - had to get out. After spells in Paris and London she ended up in the United States.

After the war she returned to Europe as a child welfare officer working for the UN: her first assignment was the care of child prisoners from Dachau. By 1948, she had married an American, Vogue photographer Don Honeyman, and after stints in New York and Paris moved with him in 1958 to London, where she still lives.

She was by then already working as a journalist - one who was, as she has put it in her own words, "passionately interested in two subjects: the Third Reich and troubled children".

That interest is unabated, to the extent that she will put herself as close to those subjects as she possibly can. Some commentators believe that closeness has distorted Sereny's moral perspective.

"I think Gitta Sereny is confused when it comes to the issue of moral blame," says Andrew O'Hagan, himself author of The Missing, a personal account of violence and murder in Britain.

"She found it very easy to empathise with Albert Speer, she found it quite easy to enter into complicity with Mary Bell in her more self-redeeming aspects - but not at all empathetic or forgiving with the mothers of the Liverpool boys who murdered Jamie Bulger, making no allowance for the economic and social horror of their lives."

Some will offer (partial) corrections to the view that, in the case of Mary Bell revisited, Gitta Sereny has flown too close to the flame. As Ian Jack, who has edited her for Granta magazine, puts it: "In her position, as a writer, it would be difficult to avoid doing the same thing. But it's a hard thing to justify on any higher level except that of writing. Writers write, that's what they do, in that sense it's an amoral activity."

Is it? That is the question which will exercise reviewers of Cries Unheard when it is published next week. It is one that has exercised many writers previously, from W. B. Yeats, Seamus Heaney and Paul Muldoon (the Troubles) to Primo Levi, Elie Wiesel and Geoffrey Hill (the Holocaust).

What marks the current furore out is its very British domesticity - the fact that the parents of Mary Bell's victims are still alive and feeling the pain she so irrevocably caused.

Whether Gitta Sereny's virtues as a writer, for which many editors and thousands of readers will vouch - her tough wisdom, scrupulous handling of fact and careful modulation of her relationship with principal interviewee - will see her through in Cries Unheard remains to be seen.

One thing is certain - you need only read the books to feel it - that every time she goes "into that darkness", Gitta Sereny is as aware as her opponents of the responsibilities which the journey entails. Now, as the titles of those books come back to haunt her, you can be sure that Sereny will have her own "battle with truth".

Cries Unheard is published in the UK by Macmillan on May 7th. Giles Foden's novel about Idi Amin, The Last King Of Scotland, was published by Faber and Faber last month.