Possible Clues:

• U.N. Secretary-General Hammarskjold
• Former UN Secretary General ___ Hammarskjöld
• The U.N.'s Hammarskjöld
• Hammarskjšld of U.N. fame
• First name among U.N. leaders
• U.N.'s Hammarskjšld
• Former U.N. chief Hammarskjold
• Hammarskjold of the United Nations
• Former UN leader Hammarskjöld [Note: The circled letters comprise a three-word instruction that, when applied to one square in the completed grid, reveals a group who might follow a code of silence]
• Mr. Hammarskjold