Americans who lived through the 9/11 attacks may not remember al-Zawahri's name, but many know his face, more than two decades on: a man in glasses, slightly smiling, invariably shown in photos by the side of bin Laden as the two arranged the strike on the United States.
An Egyptian, al-Zawahri was born June 19, 1951, to a comfortable family in a leafy, drowsy Cairo suburb. Religiously observant from boyhood, he immersed himself in a violent branch of a Sunni Islamic revival that sought to replace the governments of Egypt and other Arab nations with a harsh interpretation of Islamic rule.
Al-Zawahri worked as an eye surgeon as a young adult, but also roamed Central Asia and the Middle East, witnessing Afghans' war against Soviet occupiers in that country, and meeting young Saudi Osama bin Laden and other Arab militants rallying to help Afghanistan expel Soviet troops.
He was one of hundreds of militants captured and tortured in Egyptian prison after Islamic fundamentalists' assassination of President Anwar Sadat in 1981. Biographers say the experience further radicalized him. Seven years later, al-Zawahri was present when bin Laden founded al-Qaida.
Al-Zawahri merged his own Egyptian militant group with al-Qaida. He brought al-Qaida organizational skill and experience — honed underground in Egypt, evading Egyptian intelligence — that allowed al-Qaida to organize cells of followers and strike around the world.