The police have arrested four ultraconservative Islamists who are suspected of being accomplices to the assassination of the leftist leader Chokri Belaid this month, but they have not arrested the killer himself, Interior Minister Ali Laarayedh said Tuesday. Earlier reports citing unnamed sources had said the assassin was under arrest. Mr. Laarayedh said the police believed they had identified the killer as a hard-line Islamist who is still on the run.