A U.S. Predator drone flies through the night sky over Kandahar Air Field in Afghanistan. Such drones have been targeting groups such as the Haqqani network that are accused of attacking U.S. and Afghan forces. Kirsty Wigglesworth/AP hide caption

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Kirsty Wigglesworth/AP

Pakistani Air Force personnel guard an air base northwest of Islamabad earlier today. It was attacked Thursday by militants armed with guns, rocket launchers and suicide vests. Aamir Qureshi /AFP/Getty Images hide caption

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President Obama and John Brennan, his top counterterrorism adviser, in the Oval Office on Jan. 4, 2010. Brennan is a key voice about who gets put on the "kill list." Pete Souza/White House hide caption

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Pete Souza/White House

A 'Macabre' Process: Nominating Terrorists To Nation's 'Kill List'

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U.S. Attorney General Eric Holder (right) and FBI Director Robert Mueller announce a plot had been foiled involving men allegedly linked to the Iranian government to kill the Saudi ambassador to the U.S. and bomb the embassies of Saudi Arabia and Israel in Washington at a news conference October 11, 2011 in Washington, DC.

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Win McNamee/Getty Images

A fireman walks amongst the rubble and the smoldering wreckage of the World Trade Center in New York in the wake of the Sept. 11, 2001, attacks. Gary Friedman/AFP/Getty Images hide caption

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Gary Friedman/AFP/Getty Images