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Glenn, I believe there is a much higher imperative that Brian Ross be compelled to reveal his four Bentonite sources than you or Kevin Drum discuss.
In federal law, there is no journalistic privilege shielding a reporter from revealing a source who is a material witness in a criminal investigation, especially a capital murder case. For that reason, it's immaterial whether the sources appear to have provided false imformation. For that reason, Judy Miller was held in contempt, spent months in prison, and the US Supreme Court refused to hear her case.
If I understand correctly, the government's theory is that the anthrax came from the same lab that tested the samples recovered from the letters. The natural inference is Ross's four sources either worked at that lab or received their (mis)information from someone at the lab. Assuming the goverment's theory, the Bentonite story appears designed - at very least - to direct suspicion away from the true source of the anthrax.
That is prima facie evidence of obstruction of justice and a strong inference that Ross's sources are material witnesses, basis for a subpoena for him to reveal their identities.
Although grand jury proceedings are sealed, I think we have to assume Ross has not provided his sources to DoJ, otherwise I think ABC would have relied on that as their basis for denying your requests to reveal the sources.
While I agree with the rest of your well-reasoned article, I would suggest that the "material witness" case is the more compelling argument regarding Brian Ross's sources.