The dossier is being confirmed piece by piece and it was just revealed today that Mueller's team has traveled to London to interview Steele. Sounds like Mueller and his team of prosecutors has a solid interest in the details.
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There's no need to wonder why the special counsel is still working on this. There's a mountain of evidence just in trump's tax returns alone. Then there's the dossier, the collusion on the republican platform, the hacks on the election itself and Jr.'s emails. Mueller has a wealth of information to work with and his wealth doubles every week, it seems.
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Indictments soon.