(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."
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.
In his message, he claimed victory against the United States in Iraq and Afghanistan. He called on terrorists to continue the battle on American soil.
Al-Zawahiri named the Boston Marathon bombings on April 15 as an example of such an attack.
He encouraged his followers to provoke the United States into spending more on security, in order to "bleed America economically."
In August the Obama administration closed 19 embassies and consulates across the Middle East and North Africa after intercepting communications between al Qaeda leaders indicating possible strikes on U.S. interests.
In a message between al-Zawahiri and a top ally in Yemen, Nasir al-Wuhayshi, the leader of Al Qaeda in the Arabian Peninsula, the terror chief told the Yemeni commander to "do something," which U.S. officials inferred to mean an attack.
In his audio message Thursday, Zawahiri also claimed victory over the United States in Yemen.
But extensive drone attacks there allegedly carried out by the United States over the past two years have whittled away at al Qaeda's infrastructure and killed key leaders, diminishing its ability to carry out attacks.