The murder of a boy accused of blasphemy has come to symbolise concerns about the power of Islamist radicals in Syria's armed uprising. Paul Wood reports from Aleppo on how Sharia is spreading in rebel-held areas.

Mohammed Qataa's mother wanders the streets of Aleppo looking into strangers' faces as she tries to find her son's killers.

She knows she would recognise them. She was looking right at them when, in front of a dumbstruck and terrified crowd, Mohammed was shot dead, accused of blasphemy.

She remembers Mohammed as a happy, dutiful son, well known and well-liked in the Shaar neighbourhood where the men of the family scrape a living with a coffee cart.