Tunis (Reuters) - Islamist militant gunmen opened fire on a Tunisian customs team near the Algerian border, killing one customs agent and wounding three more, a security source said on Monday. The attack on a customs vehicle late on Sunday was in Bouchbka, in central Kasserine province, where Islamist militants belonging to Okba Ibn Nafaa, an al Qaeda-tied group, have often attacked police, army and local officials.