Al-Zawahiri has played a defining role in al Qaeda for more than a decade as Osama bin Laden's deputy. Even before the announcement, he had been widely regarded as the organization's de facto leader.
The announcement, dated June 2011 but which surfaced Thursday on the sites, said the decision to appoint al-Zawahiri was made to pay respect to the "righteous martyrs" and to honor the legacy of bin Laden.
