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.

He was 14 years old, but with no schooling possible because of the war he was usually to be found on the busy main thoroughfare through Shaar, selling the thick, sweet coffee they prefer here.

One day last month, someone asked him for a free cup. "Not even if the Prophet himself returns," he had replied, laughing. That remark was a death sentence.