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I could be wrong about this, but as a prosecutor this sounds to me as if DoJ has moved from regarding Tamm as a possible defendant to a potential witness.
This may be a logical conclusion, but do Federal prosecutors normally inform a person being investigated by a grand jury that they are no longer a target by implying the will be subpoenaed, or is there a more formal process?
Also, I don't see the inclusion of the word "voluntary" in the letter (that you bolded) quite as sinister as you appear to. Obviously, he can't be compelled to incriminate himself in the grand jury. He can, however, be compelled to testify (if a judge agrees he is a material witness to a crime) but only if he is granted immunity.
Do you think it is more likely that the DOJ is going to grant Tamm immunity or that a federal prosecutor is just making vague threats and hoping the defendant is unaware of the law? My experience is that vague threats that play off of a person's lack of legal knowledge is law 101.