With at least 67 people killed, the murderous assault on the Nairobi shopping centre has brought the terrorist group al-Shabab to world attention. A gang, armed with machine guns and grenades, stalked the mall singling out non-Muslims for execution - Kenyans and foreigners alike. Pregnant women and children were among those murdered in the name of Jihad. With its roots in war-torn Somalia and formally allied to al-Qaeda since last year, al-Shabab is banned in the US and the UK. Panorama reporter Peter Taylor was already in Kenya as final preparations for the attack were underway, investigating the network the terror group uses to cross the Somali-Kenyan border at will and its appeal to radicalised Muslim youths in Britain and beyond.