At least four Britons are among the dead in the carnage at a Nairobi shopping centre carried out, it seems, by al-Shabaab, the Somali Islamist group. We do not yet know whether there are Britons among the killers, as tweets allegedly from al-Shabaab suggest, nor whether the so-called White Widow, a Briton, was involved. What is dispiriting is that neither would be wholly surprising. Although the Prime Minister has said the killers are not motivated by Islam, we do know from Syria and elsewhere that a small minority of British Muslims have fallen prey to Islamist extremism, while Somalia remains a failed-state stronghold of jihadism in east Africa.