FEW killers have intrigued more amateur sleuths and experts than Mary Ann Cotton. Hanged in Durham Prison in 1873 for the murder of her stepson Charlie, she protested her innocence to the end.

She was reviled by those who believed she was guilty of the deaths of many people close to her. Others took a kinder view and considered her unlucky, touched by some unknown curse.

Northumbria detective Stephanie Yearnshire has carried out exhaustive research on Cotton to determine whether she was a wilful mass murderess or misunderstood matriarch, but comes down firmly on the side of the former.

"She was calculating and quite vicious. There was plenty of evidence that she had become an adulteress, bigamist, forger, fraudster, con woman, thief, inveterate liar, child abuser, lazy and vain," she says.

"There were 21 people close to Mary Anne who died. I would not subscribe to her wilfully causing all the deaths, some were clearly due to illness and diseases of the time. But, at some stage, based on more than the balance of probability, she took over the hand of fate in a number of people's lives."

There was her mother, for example, who Mary Ann visited in Seaham in March 1867, remarking to unbelieving neighbours that she thought her mother might die. Nine days later, she was indeed dead.

In the spring of 1867 there were three deaths in her household. In each case there was reported foaming at the mouth and retching after being given a drink. Again, death certificates recorded the cause of death as gastric fever.

She was finally arrested after her remaining stepson, Charlie, died in July 1872. An initial inquest verdict of death by natural causes was overturned when arsenic was found in Charlie's stomach contents, setting in train one of the most celebrated murder trials the region had seen.

Mary Ann's line of defence was that the green wallpaper in Charlie's room was heavily impregnated with arsenic. But the court had heard that Mary Ann had once been a nurse in Sunderland, where she learned all about drugs and poisons and the jury, filled with stories of her bigamy which newspapers had exposed, found her guilty of murder on March 8, 1873. She was hanged in Durham Jail in 1873.