5. Who is next?

The operatives of the 9/11 era are largely dead or captured. And key operatives like Khalid Sheikh Mohammed — an alleged 9/11 mastermind — are detained at Guantanamo Bay.

Topping the list of still-wanted is Ayman al Zawahiri, the leader of al Qaeda. With a $25 million reward on his head, Zawahiri is believed to be hiding in Pakistan.