At least 12 people were killed on Tuesday when an explosion tore through a bus full of Tunisian presidential guards in an attack that one source said was probably the work of a suicide bomber.<br />Ambulances rushed wounded from the scene and security forces closed off streets around Mohamed V Avenue, one of the main streets in the capital Tunis, where the charred wreckage of the bus lay, not far from the Interior Ministry.<br />It was the third major attack in Tunisia this year, after an Islamist militant killed 38 foreigners at a beach hotel in the resort of Sousse in June, and gunmen killed 21 tourists at the Bardo Museum in Tunis in March.<br />Fighting Islamist militants has become a major challenge for Tunisia, a small North African country that was hailed as a blueprint for democratic change in the region after an uprising in 2011 ousted autocrat Zine Abidine Ben Ali.
