An 11-year-old girl called Mary Bell was sentenced to a life of detention on this day in 1968, after being found guilty of the manslaughter of two boys aged just three and four.

Jurors at Newcastle Assizes heard from psychiatrists that the young girl was suffering from diminished responsibility and as a result found her not guilty of murder, but in sentencing Mr Justice Cusack warned of a "very grave risk to other children if she is not closely watched".

Martin Brown, aged four, of Scotswood, Newcastle-upon-Tyne, was strangled in a derelict house on 25 May. Some time later, Mary and her friend Norma Bell (no relation) broke into a deserted nursery in the area and left notes claiming responsibility for Brown’s death.

Brian Howe, three, also of Scotswood, was asphyxiated on wasteland near his home on July 31.  Norma had been present but left the scene when Mary would not listen to her pleas to stop hurting the boy. Later, Mary returned to his body to carve an ‘M’ on his stomach with a pair of scissors.

Police interviewed around 1,200 children in the area, and became suspicious of Mary and Norma, who was aged 13, when they gave evasive answers and changed their stories. Eventually, each accused the other of Brian Howe’s death.

Bell’s mother Betty was a prostitute who was said to have often left the family home to work. Mary would later claim to have suffered sexual abuse from the age of four, and by nine was being forced to take part in sexual acts with her mother’s clients.

Mary was described as having a very worldly attitude and being remarkably self-possessed, and as much the more mature of the two girls despite being the younger. By contrast Norma, who was acquitted, appeared confused and over-awed during the proceedings.

Bell was sentenced to be detained at Her Majesty’s pleasure. She spent eight years in young offenders’ institutions before being transferred to open prison. She was released in 1980 and given a new identity.