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The official has been identified by attorneys and four current and former White House officials as John Hannah, a senior national security aide on loan to Vice President Dick Cheney from then-Under Secretary of State for Arms Control and International Security Affairs John Bolton.
Hannah worked with Libby on the issue of weapons of mass destruction as part of an informal team known as the "White House Iraq Group." Hannah told friends last year that he was worried he might be implicated by the investigation, according to a report in the Washington Post.
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A month or so after obtaining testimony from Hannah and more than a dozen other senior White House officials who may have been involved in the leak, Fitzgerald sent a letter to his boss, then-acting Attorney General James Comey, seeking confirmation that he had the authority to investigate and prosecute individuals for additional crimes that may have been committed during the probe.
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http://www.truthout.org/docs_2006/040306Z.shtml