_The first man dubbed al-Qaida's No. 3, Mohammed Atef, was killed by a U.S. airstrike on Kabul in November 2001 as the Taliban regime crumbled in Afghanistan.
_Abu Zubaydah, the next to assume the role, was captured March 28, 2002, in the eastern city of Faisalabad. The Saudi-born Palestinian survived gunshot wounds in the stomach, groin and leg, and has been in U.S. custody ever since.
_Zubaydah's replacement, Khalid Shaikh Mohammed, was arrested in Rawalpindi, near the Pakistani capital, on March 1, 2003. He also is in U.S. custody.
_Ramzi Binalshibh, another top bin Laden deputy, was arrested in the southern city of Karachi on the first anniversary of the Sept. 11 attacks.
Of course, by now, all the current or potential No. 4s have already volunteered for less dangerous work, such as putting together roadside bombs.