On 27 July 1947, a 13-year-old girl reported to police that a boy called John had assaulted her by putting his hand over her mouth and saying, "What would you do if I killed you? I have done it before." This incident was not connected to Straffen until later. Six weeks later, Straffen was found to have strangled five chickens belonging to the father of a girl with whom he had quarrelled. When arrested, he was also under suspicion for burglary and, in his police interview, cheerfully confessed to it and many other incidents to which he had not been connected. Straffen was remanded in custody and the Medical Officer of HM Prison Horfield examined him, certifying that he was mentally retarded. On 10 October, he was committed to Hortham Colony in Bristol under the Mental Deficiency Act 1913.