You seem to be backwards on all of this. Mueller doesn't seem to have anything which is why it appears he's throwing it in front of a grand jury. He's subpoenaed documents and people to testify in front of the grand jury so that he can determine if there's anything substantial. It looks like he's trying to investigate it with sworn testimony.

I've see rock-solid cases go in front of grand juries, and I've seen absolutely innocent people have their cases thrown in front of the jury. Being subjected to the grand jury is no reflection on the amount of evidence present. The fact that you think you know what he has is a bit surprising. For all we know, Mueller is simply using the grand jury as a scapegoat. He may not have anything at all, knows it, and will use the grand jury as the reason Trump / Trump Jr. wont be charged... so that there isn't negative political feedback. Remember, Mueller doesn't need a grand jury to charge Trump / Trump Jr. if he had enough to charge him.

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