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.
You will note that the Rosenstein letter also directs the special Prosecutor's office to investigate "any other matters within the scope of 28 C.F.R. § 600.4(a)".
28 C.F.R. § 600.4(a) (reference attached) states, in part:
a. "The jurisdiction of a Special Counsel shall also include the authority to investigate and prosecute federal crimes committed in the course of, and with intent to interfere with, the Special Counsel's investigation, such as perjury, obstruction of justice, destruction of evidence, and intimidation of witnesses; and to conduct appeals arising out of the matter being investigated and/or prosecuted.
b. "If in the course of his or her investigation the Special Counsel concludes that additional jurisdiction beyond that specified in his or her original jurisdiction is necessary in order to fully investigate and resolve the matters assigned, or to investigate new matters that come to light in the course of his or her investigation, he or she shall consult with the Attorney General, who will determine whether to include the additional matters within the Special Counsel's jurisdiction or assign them elsewhere. "
With the recusal of the AG, the Deputy AG is ultimately responsible, so I believe that Mueller does have the power to investigate anyone outside the Trump Campaign. Of course, if there is a real issue with this, the US supreme court is always available.
see:
https://www.law.cornell.edu/cfr/text/28/600.4
