The letter, which was signed by Ayman al-Zawahiri, al-Qaeda’s second in command, was written after the American embassy in Yemen was attacked by simultaneous suicide car bombs in September.

Western security officials said the missive thanked the leadership of Iran’s Revolutionary Guards for providing assistance to al-Qaeda to set up its terrorist network in Yemen, which has suffered ten al-Qaeda-related terror attacks in the past year, including two bomb attacks against the American embassy.