Complex sentence: A secret letter, delivered directly to Khatami by Sultan Qaboos of Oman, stated that the United States had evidence of direct Iranian involvement in the act, and demanded that those involved be held responsible for their actions.
Split into simpler sentences: A secret letter was delivered directly to Khatami by Sultan Qaboos of Oman. The letter stated that the United States had evidence of direct Iranian involvement in the act, and demanded that those involved be held responsible for their actions.