Afghan security forces and British soldiers inspect the site of a suicide attack in the heart of Kabul, Afghanistan, on Saturday. Three American civilian contractors are reportedly among the dead. Massoud Hossaini/AP hide caption

itoggle caption Massoud Hossaini/AP

Afghan security forces inspect a suicide bombing attack on Aug. 10 near the main gate of Kabul's international airport. It was one of a series of recent attacks in the Afghan capital that have left at least 50 dead. Violence in Afghanistan has increased since U.S. combat troops pulled out last year. Massoud Hossaini/AP hide caption

itoggle caption Massoud Hossaini/AP

A 1998 file photo of Ayman al-Zawahri speaking at a news conference in Khost, Afghanistan. The reclusive al-Qaida leader says he has pledged support to the new chief of the Taliban, Mullah Akhtar Mansoor. Mazhar Ali Khan/AP hide caption

itoggle caption Mazhar Ali Khan/AP

This photo provided by the U.S. Government, presented during a trial, shows a photo of Irek Hamidullin surrendering after being wounded by U.S. forces in Afghanistan on Nov. 29, 2009. AP hide caption

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Afghan President Ashraf Ghani at the Blair House in Washington, D.C. Ghani will be meeting with President Obama this week. Emily Jan/NPR hide caption

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Commander of the International Security Assistance Force (ISAF), Gen. John Campbell opens the "Resolute Support" flag during a ceremony at the ISAF headquarters in Kabul on Sunday. Massoud Hossaini/AP hide caption

itoggle caption Massoud Hossaini/AP

Afghan security personnel inspect a damaged bus at the site of a suicide attack in Kabul, Afghanistan, on Saturday. There was no immediate report of casualties, but the attack was one of several in the last 24 hours that have been blamed on the Taliban. Rahmat Gul/AP hide caption

itoggle caption Rahmat Gul/AP