Lara Logan reports on evidence which suggests that Ayman al Zawahiri, the second most powerful leader in al Qaeda directly behind Osama Bin Laden, may have been critically wounded and possibly dead.
CBS News has obtained a copy of an intercepted letter from sources in Pakistan, which urgently requests a doctor to treat al-Zawahiri. He's believed to be somewhere in Pakistan's remote tribal areas of Pakistan.
The letter refers to Sheikh Dr. Ayman al-Zawahiri by name - and says that he is in "severe pain" and his "injuries are infected."
However, CBS News has not been able to confirm the authenticity of Baitullah Mehsud's signature and seal.
The letter is dated July 29 - one day after a U.S. air strike that killed al Qaeda weapons expert Abu Khabab al-Masri, and five other Arabs in South Waziristan.