(CNN) -- Americans commemorated this week the loss of those who died at the hands of al Qaeda terrorists on September 11, 2001. Their leader chimed in a day later with new threats against the United States. Ayman al-Zawahiri called on his followers in an audio message posted on the Internet on Thursday to "land a large strike on it, even if it takes years of patience for this." 9/11 anniversary: 'Our hearts still ache,' Obama says Al-Zawahiri has headed al Qaeda, since a U.S. military operation killed his predecessor and al Qaeda's founder, Osama bin Laden, in May 2011 in Pakistan.
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To commemorate 9/11, al Qaeda's leader issued new threats
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Ayman al-Zawahiri called on his followers to "land a large strike"
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He named the Boston Marathon bombings as an example
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Drone strikes have weakened al Qaeda in Yemen
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But extensive drone attacks there allegedly carried out by the United States over the past two years have whittled away at @placeholder's infrastructure and killed key leaders, diminishing its ability to carry out attacks.