Her daughter, Mandy Honeyman, confirmed the death.  A resident of London who had lived most recently in Cambridge, Ms. Sereny (pronounced suh-REE-nee) was long considered one of the foremost investigative journalists in Britain. She was known for her tenacity, her willingness to sit down with some of the most reviled figures of the era and her long, probing interviews — sometimes spanning years — that could prompt her subjects to reveal extraordinary things about themselves.  Among her best-known books are “Into That Darkness” (1974), about Franz Stangl, the commandant of the Sobibor and Treblinka extermination camps, who was responsible for the deaths of 900,000 people; “Albert Speer: His Battle With Truth” (1995); and “Cries Unheard: Why Children Kill,” about Mary Bell, an Englishwoman who as a girl in the 1960s murdered two young boys — a book that caused a furor when it was published in Britain in 1998.  Acclaimed by some reviewers and criticized by others, Ms. Sereny’s books were as much psychological studies as historical ones. As her work made plain, she was interested less in plumbing the “what” of history’s evil deeds than she was in the “why” of their perpetrators. Few people if any, she often said, were born evil; instead, she argued, they were made that way by traumatic conditions that could generally be located in childhood.  What interested her above all was conscience. Throughout Ms. Sereny’s writing, the questions that arose again and again were whether history’s great malefactors were capable of genuine remorse, and whether, for them, there existed even the faint possibility of redemption.  For her psychobiography of Stangl, Ms. Sereny interviewed him for more than 60 hours in prison in Düsseldorf, Germany, where he was serving a life sentence. On the last day they met, Stangl, speaking of the Holocaust, told her, “In reality I share the guilt.” He died less than 24 hours later.  The biography of Speer, Hitler’s minister of armaments and war production, consumed Ms. Sereny for more than a decade. Over her many interviews with Speer, who had finished a 20-year sentence in Spandau Prison and was living in retirement, she grew to like him, a fact she acknowledged in her book.  Their amicable relationship notwithstanding, Ms. Sereny demonstrated in the book that Speer, contrary to his assertions that he had been ignorant of Hitler’s plan for the mass extermination of Jews, knew about the Final Solution as early as 1943.  “Cries Unheard” was the second of two books Ms. Sereny wrote about Mary Bell. In 1968, Mary, one day shy of 11, strangled a neighbor boy of 4; nine weeks later she strangled a 3-year-old.  Mary’s trial was a sensation in Britain. Her impassive demeanor caused the press to label her a coldblooded incarnation of evil. Convicted of manslaughter, she spent 12 years in prison before being released in 1980.  Ms. Sereny, who covered the trial for The Times of London, wrote about the proceedings in the first of the two books, “The Case of Mary Bell,” published in 1972.  The second book centered on Ms. Sereny’s long series of interviews with Ms. Bell, who was by then out of prison. Ms. Sereny recounted Ms. Bell’s description of prolonged childhood abuse at the hands of her mother, a prostitute who was said to have forced Mary, from the age of 4 on, to engage in sex acts with her clients.  Ms. Bell had long been one of the most despised people in Britain, and Ms. Sereny’s book was fiercely criticized for its seemingly sympathetic portrayal of her. Ms. Sereny was also pilloried for paying Ms. Bell a substantial sum for her participation — many accounts said £50,000.  Ms. Sereny declined to confirm the precise figure but acknowledged having paid Ms. Bell. “If I hadn’t done so, I would have made myself guilty of doing what was done to her virtually since she was born — to use her,” she wrote in a letter to the mother of one of the victims in 1998.  To critics who taxed Ms. Sereny with displaying excessive sympathy for her subjects, she replied that her mandate was neither to condone nor condemn but simply to understand — to penetrate a subject’s protective carapace.  The daughter of a Hungarian father and a German mother, Gitta Sereny was born in Vienna on March 13, 1921. Her father died when she was about 2, and she was reared by her mother, an actress. The young Ms. Sereny was educated in England and Vienna, where she studied at the drama school founded by the noted actor and director Max Reinhardt. After Germany annexed Austria in 1938, she eventually made her way to France (where she worked as a volunteer nurse with children displaced by the German occupation) and later to the United States.  At war’s end, Ms. Sereny returned to Europe as a welfare officer with the United Nations Relief and Rehabilitation Administration, trying to reunite child survivors of Dachau with their families.  Ms. Sereny’s husband, Donald Honeyman, a Vogue photographer whom she married in 1948, died last year. (She was known in private life as Gitta Sereny Honeyman.) Besides her daughter, she is survived by a son, Chris; two grandchildren; and a great-grandchild.  Her other books include “The Healing Wound” (2001), a memoiristic essay collection about wartime Europe, and “The Invisible Children” (1985), a study of child prostitution.  Though Ms. Sereny chose to spend her professional life steeped in evil, she said that her calling had little effect on her own temperament.  “I know this is difficult to believe, but I’m really, in the old sense of the word, quite a gay person,” she told the newspaper The Scotsman in 2000. “I’m very optimistic. About the world. About people. I believe the majority of people are good.”
Ms. Serenyâs books, which included portraits of Nazi concentration camp commanders and young murderers, concentrated on psychology as much as chronology.