The solution to the darkest of all Agatha Christie mysteries may be at hand. What lay behind her extraordinary 11-day disappearance in 1926? Several plausible theories have competed for favour over the years, but biographer Andrew Norman believes he is the first to find one that satisfies every element of the case.

In his study of the writer's life published this autumn, Norman uses medical case studies to show that Christie was in the grip of a rare but increasingly acknowledged mental condition known as a 'fugue state', or a period of out-of-body amnesia induced by stress. In effect, the writer was in a kind of trance for several days, he claims.

The mystery, which has puzzled both the police and Christie fans for 80 years, is a why-dunnit, rather than a who-dunnit. It began on the evening of Friday 3 December at Styles, the Berkshire home of the crime writer, by then already an established name, with a sixth novel, The Murder of Roger Ackroyd, selling well. Around 9.45pm, without warning, she drove away from the house, having first gone upstairs to kiss her sleeping daughter, Rosalind. Her abandoned Morris Cowley was later found down a slope at Newlands Corner near Guildford. There was no sign of her.

For 11 days the country buzzed with conjecture about the disappearance. All the elements of a classic Christie story were there. The Silent Pool, a natural spring near the accident scene, for instance, was said to be the site of the death of a young girl and her brother and many thought the novelist had drowned herself there. Others suggested the incident was a publicity stunt, while, more chillingly, some clues seemed to point in the direction of murder at the hands of her unfaithful husband, Archie Christie, a former First World War fighter pilot.

Such was the speculation that the home secretary of the day, William Joynson-Hicks, put pressure on the police to make faster progress. Even the celebrated crime writers Sir Arthur Conan Doyle, creator of Sherlock Holmes, and Dorothy L Sayers, author of the Lord Peter Wimsey series, were drawn into the puzzle. Conan Doyle, who was interested in the occult, took a discarded glove of Christie's to a medium, while Sayers visited the scene of the disappearance, later using it in the novel Unnatural Death.

Christie was eventually discovered safe, but in circumstances that raised more questions than they answered. Alone, and using an assumed name, she had been living in a spa hotel in Harrogate since the day after her disappearance, even though news of her case had reached as far as the front page of the New York Times. Until now the two most popular theories offered for these strange events have been that either Christie was suffering from memory loss after a car crash, or that she had planned the whole thing to thwart her husband's plans to spend a weekend with his mistress at a house close to where she abandoned her car.

But Norman, a former doctor, believes the novelist was in a fugue state, or, more technically, a psychogenic trance, a rare, deluded condition brought on by trauma or depression, which may also have led the writer and actor Stephen Fry to travel to Bruges in 1995 without leaving word with his friends or family.

'This kind of fugue state, which is much better understood these days, fits the symptoms that Christie showed during her stay in Harrogate,' said Norman.

In his book, The Finished Portrait, Norman says that her adoption of a new personality - she took the name Teresa Neele - and failure to recognise herself in newspaper photographs were signs that the novelist had fallen into a psychogenic amnesia after a period of depression. 'I believe she was suicidal,' said Norman. 'Her state of mind was very low and she writes about it later through the character of Celia in her autobiographical novel, Unfinished Portrait.'

She divorced in 1928 and later married archaeologist Sir Max Mallowan.

The Queen of Crime



The premise Agatha Mary Clarissa Miller was born on 15 September 1890. She married Archibald Christie in 1914 and in 1930 became Lady Mallowan on marriage to her second husband, Max Mallowan.

Birth of a famous Belgian Christie's first crime novel, The Mysterious Affair at Styles, introducing Hercule Poirot, was published in 1920.

As She Liked It In 1919, Christie gave birth to her only child, Rosalind, named after Shakespeare's heroine.

Facts of the case An estimated billion copies of her novels have been sold in English, and another billion in 103 other languages.

Serial murders In total, she wrote 80 novels.

The final pages She died in 1976 in Cholsey, near Wallingford, Oxfordshire.