As relatives of the two boys she killed staged a protest rally, detectives confirmed they are studying claims that Bell herself was attacked in the approved school where she was sent after her conviction at the age of 11.
Red Bank Special Unit at the Newton-le-Willows school in Merseyside is one of a string of approved schools and children's homes under investigation in the north-west of England, a spokesman for Merseyside police said.
In 1970, Bell, then 13, said she had been sexually abused by a housemaster at Red Bank where she was resident from 1969 to 1973. Her story was not believed.
But detectives are now understood to be re-examining the claims that Bell was repeatedly attacked at the unit where she was the only girl inmate.
The wider investigation, named Operation Care, was started four years ago. Detective Superintendent John Robbins said: "We cannot go into specifics at the moment. The inquiry covers offences of indecency against children in care, going back over many many years."
The first copies of Gitta Sereny's book about Bell, Cries Unheard, went on sale yesterday, except in Newcastle-upon-Tyne where the boys were killed. Protesters in Newcastle called for the book to be banned and said criminals should not benefit financially from their crimes.
Bell has been paid an undisclosed amount by the author for her co-operation. The Attorney General has meanwhile said he cannot force her to hand over any money made.
June Richardson, mother of Bell's first victim Martin Brown, and Eileen Corrigan, mother of her second victim, Brian Howe, were joined by mothers of other murdered children in protest at this legal position. Mrs Richardson said: "The law says we cannot get the money back, but ... if there is a loophole then believe me we will find it."
About 100 people signed a petition calling for legislation so criminals cannot make money from their crimes. Beverley Palmer, whose daughter, Rosie, was murdered by a paedophile, said: "Jack Straw could ban beef on the bone in a day, why can't he ban this book?"
Many protesters expressed sympathy for the 14-year-old daughter of Mary Bell, who knew nothing of her 41-year-old mother's past until the row over the book broke last week.
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