If he’s still employed at at FBI, Congress can call him on the carpet at any time to compel him to answer...
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I do not know if Baker is an agent, but I have worked with many over the years and those trained to work undercover are trained in the art of deception and lying. Their lives working U/C depend on it. So, good luck thinking an oath is going to get you the truth.
It's not that they can compel him to tell the truth; it's that they can compel him to *testify*.
Mueller ain't the only one who can use perjury traps: which is why Rush Limbaugh had this ditty about Hillary: