Mary Bell The U.K. was rocked by the case of Mary Bell, an 11-year-old girl who in 1968 became one of the youngest serial killers in history. Mary Bell was convicted of killing two small boys in such a way that left the nation stunned. Authorities recovered her first victim, 4-year-old Martin Brown, from an abandoned house, and later, they discovered 3-year-old Brian Howe strangled and mutilated in a shallow grave. Bell's background was troubling.