Mueller's sole job is to find out what happened (with regards to Russian collusion) and follow all trails regardless of where they may lead.
This is not accurate. If you read the document by Rod Rosenstein that sets the parameters for the Mueller investigation it says that Mueller is to investigate any possible collusion between Russia and the Trump campaign and any related matters. It says nothing about investigating collusion by anyone outside the Trump campaign. Thus, Mueller is not empowered to investigate the Clinton campaign, DNC or others that worked with Russian sources to get dirt on Trump. Ol' Rod was pretty slick in the way he set this up.
