Mueller is presumably investigation foreign intervention into our elections, wherever that may lead. The FBI is more than just a law enforcement agency; it is an intelligence service. Normally the FBI would do it, but with Comey's firing and questions if those connections would lead to the executive they are likely now viewed as tainted. Having said that there may have been crimes committed there around things like hacking, that may or may not have been committed by citizens.

The question of foreign meddling into our election is all tied up with politics now which is why an independent is now involved. How independent that guy really is remains to be seen.

Maybe the question that needs to be properly answered first is - Is the United States at war with Russia?
And please do not quote Clausewitz's aphorism - "War is the continuation of politics by other means." Doing so is a copout and also means that you are not able to support your contention or argument with factual elements of information; or you are just plain intellectually lazy.

Robert Mueller is investigating classified information and testimony that are showing probable cause for collusion with Russia... which would start the ball rolling on a whole world of hurt for Trump. Here are some very probable crimes that Trump may face: Treason(collusion w/Russia to betray our election process), Obstruction of Justice(Firing Comey and asking NSA and intelligence officials to cease the Flynn investigation), Perjury (Lying under oath... which he would likely do if he ever had to testify because he can't tell the truth to save his life), and Contempt of Congress (Holding out requested documents/tax returns after pleading the 5th... because he would also do this because he has too much to hide and thinks he is above the law as president). Let's also put a cherry on top and say that Trump is even hiding Tax Evasion in his withheld tax returns. All that could come to bear because of this ongoing classified investigation that will likely lead to his impeachment trial.