After three years in prison, during which he was tortured, he joined bin Laden in Afghanistan in 1986 as his personal physician and a mentor in terrorism.
"Ayman Zawahiri and Osama bin Laden [have] the same ideology — to defend in their terms their religion and their land," says Dia'a Rashwan, an analyst at the Al Ahram Center for Strategic Studies.
In 1998, Zawahiri was one of five Islamic leaders to sign on to bin Laden's declaration calling for attacks against U.S. citizens. More broadly, he was believed to be the person who most influenced bin Laden years ago to take up a worldwide struggle against perceived enemies of Islam. According to some analysts, Zawahiri helped turn bin Laden from a financial backer of the Afghan resistance into a strong believer in the ideology of jihad.
"For his cause, he would embrace death more than we would embrace life," says Salah in Arabic. "He doesn't care if he dies."
The United States Department of State is offering a $25 million reward for Zawahiri, who was once trained to save lives but is wanted for killing thousands instead.
ABCNEWS' Jim Wooten, Brian Ross, David Scott and Michael Baltierra contributed to this report.