A web posting Thursday said Zawahri has assumed responsibility for the group, and that there will be no shift in al-Qaida policy.  He had been the number two. Although some terror analysts speculated he lacked bin Laden's potent charisma and ability to hold al Qaeda together.

Born into a wealthy family in Cairo, al-Zawahiri is a physician and founding member of the Egyptian Islamic Jihad, a militant organization that opposed the secular Egyptian government of Hosni Mubarak and sought its overthrow through violent means. Like bin Laden, al-Zawahiri also went to Afghanistan during its fight against the Soviets, although he was there primarily to offer medical expertise. Al-Zawahiri's wife and three children were killed in December 2001 in a U.S. attack on the family's residence in Afghanistan.

An associate of bin Laden's since the 1980s, he has been described as the organizational mastermind of the operation, complementing bin Laden's inspirational and financial role. Zawahri released a message earlier this month acknowledging bin Laden's death.  In it, he vowed to continue the path of jihad to expel what he called invaders of Muslim land and to purify it from injustice.

The FBI is offering a $25 million reward for information leading to his capture, the same amount as the reward for bin Laden. Zawahiri has been in hiding since the September 11, 2001 terror attacks on the World Trade Center.

Previously, Zawahiri was jailed for three years in Egypt for militancy and was implicated in the assassination of Egyptian president Anwar Sadat in 1981, and a 1997 massacre of tourists in Luxor, AFP reported.

Facing a death sentence, he left Egypt in the mid-1980s initially for Saudi Arabia, but then headed for Pakistan's northwestern city of Peshawar and then to Afghanistan, where he joined forces with bin Laden.