Senior Pakistani officials tell ABC News they believe Zawahiri and 28-year-old California native Adam Gadahn spent several days together. He was reporteldy seen in late August north of the Pakistani city of Karachi.
A federal grand jury in California indicted Gadahn for treason, alleging he gave "al Qaeda aid and comfort...with intent to betray the United States."
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Gadahn, a one-time heavy-metal music fanatic who converted to Islam as a teenager, has become a leading propagandist for Osama bin Laden, according to U.S. intelligence officials.
He first surfaced in an al Qaeda video in 2004, using the nom de guerre "Azzam the American." He spoke perfect English and warned "the streets of America shall run red with blood."
In another tape released last month, Gadahn urged American troops in Iraq and Afghanistan "and wherever else 'dubyah' has sent you to die" to desert the U.S. military and "join the winning side."
Gadahn could face the death penalty if convicted of treason, but officials have to find him first.
He was last spotted in August in the South Waziristan region of Pakistan, where the central government has little control. He moved before he could be captured.
Gadahn already faces charges in the U.S. for allegedly providing material support to terrorism, and law enforcement officials say if he is captured inside Pakistan, the indictment lays the groundwork for him to be turned over to U.S. custody.
In addition to his propaganda role, Gadahn is believed to be helping al Qaeda with English translations and understanding American culture and vulnerabilities.