Jack the Ripper is caught,' Patricia Cornwell wrote at the end of her 2002 book, Portrait Of A Killer. 'We have done it together.'

Cornwell is the bestselling author of the Kay Scarpetta mystery series. There are parallels between the creator and the creation. Like Kay Scarpetta, Cornwell knows her forensic science (though her heroine outstrips her); like Scarpetta, she has considerable means; like Scarpetta, she believes that an unsolved murder is an abomination; like Scarpetta, she has battled alcoholism and come out on top.

Her books are respected for their accuracy of the forensic detail, though long-time Scarpetta fans are dismayed at her increasingly tortuous plotting.

In 2002, Cornwell put her reputation on the line -- not as the creator of a detective, but as a writer turned detective herself. She claimed to have solved the most notorious series of murders ever; she claimed she knew the identity of Jack the Ripper, the man who murdered at least seven women in the Whitechapel area; she claimed the Ripper was the painter Walter Sickert.

Over the next three years, Ripperlogists tore her theory to shreds. While Cornwell had clearly spent time, money and passion on the research, her case seemed unconvincing. Though many doubted her theory about the painter, no one doubted that she believed that theory in all sincerity.

When I read Portrait Of A Killer three years ago, I read it as a book about Jack the Ripper, and looked to see if Cornwell had actually assembled enough evidence. It seemed to me that she was after the wrong suspect, and her case was not convincing at all, though it was passionately argued.

This week, Cornwell is back with what she says is near-conclusive evidence that will go into the revised edition of Portrait Of A Killer. To prove her lack of obsession with Sickert, the crime writer took out full page ads in The Guardian and The Independent that declare she isn't obsessed with him.

I picked up Portrait Of A Killer again, realising I had read it wrong: this was not the story of a hunt for a long-dead killer. It was a study in obsession.

Many crime writers know the impulse that takes you down a cold, long-dead trail. P D James did this in The Maul And The Pear Tree, where she tried to exhume the story of the Ratcliffe Highway murders. She and her co-writer set out, like Cornwell, impelled by a need to deliver a belated justice; but in the end, James conceded she could not name the killer. The investigation was its own catharsis; she moved on.

Crime writer James Ellroy's mother was murdered when he was a child, and the killers were never found. At 32, he wrote Clandestine, a thinly veiled account of his mother's murder in which he made his father the killer, even though he knew that was neither possible nor true.

Then he wrote the Black Dahlia books, about the unsolved murder of a young girl, summing up his reasons cynically: 'Boy, bereft, seizes on Black Dahlia murder case to express the grief he never felt on the occasion of his mother's death.' It would take him more years, and more books, before he found a kind of resolution.

Even Ellroy didn't display the degree of obsession that Cornwell does with Sickert. 'It has always been easier for me to get angry than to show fear or loss, and I was losing my life to Walter Richard Sickert. He was taking it away from me,' Cornwell wrote.

At lectures, Ripper fans watched in bemusement as Cornwell's paranoia grew --she hired armed guards to protect her from possible attacks from Ripperologists, she called anyone who cast doubts on her theories part of the 'Klingon brigade'.

Patricia Cornwell has one more chance, with the second edition of Portrait Of A Killer.

If she has worked out her forensic evidence, if she has made the case beyond reasonable doubt, then she'll have got what she wanted: banner headlines saying 'Crime novelist solves Ripper mystery'.