In December 1968 two girls who lived next door to each other - Mary, aged eleven, and Norma, thirteen - stood before a criminal court in Newcastle, accused of strangling two little boys; Martin Brown, four years old, and Brian Howe, three.

Norma was acquitted. Mary Bell, the younger but infinitely more sophisticated and cooler of the two, was found guilty of manslaughter. She evaded being branded as a murderer due to what the court ruled as 'diminished responsibility', but she was sentenced to 'detention' for life.

Step by step, Gitta Sereny pieces together a gripping and rare study of a horrifying crime; the murders, the events surrounding them, the alternately bizzare and nonchalant behaviour of the two girls, their brazen offers to help the distraught families of the dead boys, the police work that led to their apprehension, and finally the trial itself. What emerges from this extraorindary case is the inability of society to anticipate such events and to take adequate steps once disaster has struck.