By Con Coughlin
Last Updated: 11:17PM GMT 23 Nov 2008
Delivery of the letter exposed the rising role of Saad bin Laden, son of the al-Qaeda leader, Osama as an intermediary between the organisation and Iran. Saad bin Laden has been living in Iran since the fall of the Taliban in Afghanistan in 2001, apparently under house arrest.
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.