Over the years, Zawahiri has released a drumbeat of audio and video messages via al Qaeda affiliated web sites, more than three dozen since 2003.
In his most recent message, an hour-plus long video posted online in April, Zawahiri calls on Arab armies to intervene in Libya to help eject dictator Moammar Gadhafi before "Western aid... turns into invasions."
The video was the first message in a year in a half from Zawahiri, and the first since Egypt's successful revolution. It features Zawahiri in a white robe with the barrel of an assault rifle visible at his side. In addition to discussing the bloody fighting in Libya, Zawahiri celebrates the overthrow of Egyptian President Hosni Mubarak and says that the continuation of the North African nation depends on the destruction of Israel and the establishment of Islamic law.
As a leader of Islamic Jihad in Egypt, Zawahiri had attempted to organize a coup against Anwar Sadat. Zawahiri was arrested, imprisoned and allegedly tortured by Egyptian authorities after Anwar Sadat's assassination. He served a three-year sentence on a weapons charge and moved to Saudi Arabia in 1985 on his release. After Saudi Arabia, he moved to Pakistan, where he met Bin Laden.
Prior to the April video, Zawahiri had lauded the revolts in Egypt and Tunisia in the latest issue of al Qaeda's English-language magazine "Inspire".
In February, in the midst of the Egyptian uprising, Zawahiri released a 34-minute audio message blasting the corrupt government under Mubarak.
Three years ago, Zawahiri also wrote a book that provided a religious justification for using weapons of mass destruction against America.
CIA director Leon Panetta said Tuesday that Zawahiri was "moving up very fast" on the U.S. enemies list, but noted that the U.S. needed to see for certain who would replace bin Laden as al Qaeda chief. The FBI also told ABC News it was "premature" to speculate on who would fill bin Laden's now-vacant position on the Most Wanted list.
White House counterterror advisor John Brennan said that Zawahiri lacks charisma and has enemies within al Qaeda. A former attorney of Zawahiri's in Egypt wrote a book about Zawahiri that portrayed him as abrasive, stubborn and arrogant, and said that would limit his effectiveness.