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I think we can give Glenn a pass here... Oftentimes, he'll use a few sentences or even a paragraph to elucidate just why he credits a particular source as trustworthy. In this case, I think the text of the emails presented, echoing NBC/McCaffrey/Weinerboy's talking points speak for themselves, even if the source has to remain a secret for the purpose of job security. Certainly a different case from Williams asserting trust in his consistent go-to war expert based on the shiny metal on his chest, and the fact that they like each other and hobnobbed on Pentagon flights to Iraq.
Plus, I think it's fair to apply different standards of journalistic trust to whistle-blowers in comparison to alleged experts offering "educated opinions"...