The first thing U.S. authorities must determine is how and when 20-year-old John Walker came to be among Taliban warriors and what he did once he got there. Walker will be in the most serious jeopardy if it turns out that he had anything to do with the death of CIA operative Johnny ("Mike") Spann.
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From the little that is known about Walker's case so far, legal experts say it is unlikely that he will face treason charges, for which the Constitution applies a maximum penalty of death. That's partly because the framers, mindful of how abusively treason had been applied in England, set a high standard of proof: a confession in open court, or at least two witnesses testifying to someone's waging an "overt act" of war against his country or giving aid and comfort to the enemy.
Nor can Walker be hauled in front of a military tribunal, because President Bush decreed that they try only noncitizens. Courts generally regard citizenship as a right that can be relinquished but not taken away. That means Walker's case would have to go to federal court, but nowhere in the statute books does there seem to be a law that precisely fits the crime he may have committed. He might be tried for seditious conspiracy, except that the law against it specifically applies to actions that happen within U.S. jurisdiction.
Walker may have been only a foot soldier and may not know much about Taliban leaders. But many legal observers expect Walker to cut a deal whereby he cooperates with U.S. authorities, possibly providing valuable information about the Taliban, in exchange for pleading guilty to a lesser offense. If that happens, Walker may indeed end up before a military tribunal--but as a witness rather than a defendant.