Citing unnamed sources, Newsweek magazine reports Walker was given the choice between training for terrorist instruction or fighting as an al-Qaida soldier against the northern alliance in the months before the Sept. 11 attacks.
A Justice Department official told Newsweek that Walker chose to fight - an issue now at the center of discussions about Walker's fate.
"There's still a lot of missing pieces," an official working on the case told the magazine. "We're trying to figure out exactly what he did."
Walker was captured by U.S. forces after a prison uprising at a fortress in the northern Afghan city of Mazar-e-Sharif this month.
He could face charges such as providing support to terrorists, which carries a 15-year sentence, or life imprisonment, if his support resulted in a death.
Walker could also be charged with treason, which carries a maximum sentence of death.
Bush administration officials told Newsweek that some authorities may want to delay charging Walker if he has information that could help prosecute bin Laden or other al-Qaida leaders.
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