U.S.officials have long said they believe al-Qaida leader Osama bin Laden and his top commanders shifted their base of operation from Afghanistan to Pakistan's largely autonomous tribal regions after coalition forces invaded Afghanistan in 2001.

VOA: standard.2009.10.30

Earlier this month, the Pentagon said it had received "credible reports" suggesting that Mohammed Atef -- one of al Qaeda's top aides to Osama bin Laden -- was killed in an airstrike south of Kabul.

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Some U.S. officials doubt that top Pakistani officials knew of bin Laden's location.

WSJ: U.S. to Probe Pakistan Support for bin Laden

President Bush and other top American officials have said bin Laden is the prime suspect in the September 11 attacks.

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