It’s the IG that found most likely the exculpatory evidence in some fashion and by law had to report it to DOJ which in turn runs a parallel investigation into such criminal behavior. If Mueller was investigating McCabe and knew of the alleged shenanigans and/or had doubts then I doubt he would have basically forced Flynn into a plea deal last December. In fact if Mueller did know and still accepted Flynn guilty plea then he would be in big trouble
[It was already widely known (even by prosecutor Fitzgerald) that the actual "leaker" was Richard Armitage, via columnist Robert Novak. No evidence has ever come to light that Mr. Libby disclosed Plame's CIA status to Mr. Novak.]
And yet Fitzgerald prosecuted Libby to hurt Bush.
It's what these holes do.