Al Qaeda has named a former deputy to Osama bin Laden as its new terror leader following bin Laden's death at the hands of U.S. Navy SEALs in May, the organization announced on a jihadi website overnight.
Ayman al-Zawahiri, the Egyptian-born doctor who turns 60 on Sunday, was identified in the statement as the group's new emir in a "new era" for al Qaeda -- the group he helped found with bin Laden.
Earlier this month, al-Zawahiri appeared in a video in which he vowed to avenge bin Laden's death "blood for blood." In that video, he urged his followers to remember the 9/11 terror attacks, saying the attacks "destroyed the symbol of American economy in New York and the symbol of American military might in the Pentagon."
READ: Al Qaeda Deputy Surfaces: New Terror Leader?
Al-Zawahiri was long believed to be a leading contender to take over al Qaeda, though it took the organization more than a month to announce the transition. Noman Benotman, a former al Qaeda member and close associate of al-Zawahiri's in the 1990s, told ABC News the delay is a sign there were likely disputes within al Qaeda over al-Zawahiri's leadership.
A U.S. counter-terrorism official agreed and said that an al Qaeda under al-Zawahiri's leadership may not garner the same following as under bin Laden.
"Al-Zawahiri has not demonstrated strong leadership skills in the past and he is much less charismatic than bin Laden," the official said. "Unlike many of al Qaeda's top leaders, al-Zawahiri hasn't had combat experience... It remains an open question whether al-Zawahiri will engender the same loyalty that bin Laden did."
U.S. State Department spokesperson Victoria Nuland went further, saying it didn't matter either way who was the head of al Qaeda.
"Frankly, it barely matters who runs al Qaeda, because al Qaeda is a bankrupt ideology," she said today. "If you look around the world, the peaceful movements for change around the world have done far more for Muslim people than al Qaeda has ever produced."
However, Benotman said al-Zawahiri is a master political in-fighter and a supreme organizer of war. His first order of business, Benotman said, is to "decontaminate the group's reputation in the Muslim world" -- a reputation mired by the killing of Muslim civilians and al Qaeda's lack of participation in the Arab Spring.
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$25 Million Bounty
Al-Zawahiri is wanted by the U.S. for his role in the 1998 bombings of the U.S. embassies in Tanzania and Kenya, which claimed 224 lives.
Over the years, al-Zawahiri has released a drumbeat of audio and video messages via al Qaeda affiliated web sites, more than three dozen since 2003. He has also appeared in al Qaeda's English-language magazine, "Inspire," in which he lauded the popular uprisings in the Middle East.
Before the "motherlode" of evidence recovered from the SEALs bin Laden operation appeared to show otherwise, for years U.S. officials believed bin Laden was serving more as a figurehead for al Qaeda and al-Zawahiri was its true operational commander.
He currently sports a $25 million bounty for information leading to his capture, the biggest bounty of any terrorist wanted by the U.S.
ABC News' Hanna Siegel contributed to this report.