(CBS) Ayman al-Zawahiri - the second most powerful leader in al Qaeda and Osama Bin Laden's No. 2 - may be critically wounded and possibly dead, CBS News chief foreign affairs correspondent Lara Logan reports exclusively.

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."

[Following the broadcast of this report, Taliban officials denied that al-Zawahiri was injured or killed in any missile attack.]

The letter is reportedly written by local Taliban leader, Baitullah Mehsud, whose signature and seal are visible on the letter.

The Taliban logo and the Mehsud's seal have been confirmed by experts as legitimate.

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.

U.S. authorities have said they do not have information that al-Zawahiri was present during Monday's strike, or that he was injured.

However, a counter-intelligence expert and other U.S. officials confirmed to CBS News that the U.S. is looking into reports that al-Zawahiri is dead.

Logan reports that while there have been false death rumors regarding al-Zawahiri before, there have been no denials yet from Pakistan, the U.S. or al Qaeda Web sites.

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