TUNIS - Men who opened fire on Tunisian officers in the north of the country on Wednesday were suspected members of al-Qaida, a security source told Reuters.The group of nine armed men killed four military and policemen at a checkpoint, the source said, revising an earlier figure. Three of the assailants were killed.The men were thought to be members of al-Qaida in the Islamic Magreb (AQIM), the source said.