Mueller has officially been tasked with investigating "any links and/or coordination between Russian government and individuals associated with the campaign of President Donald Trump, and any matters that arose or may arise directly from the investigation."

Although the investigation specifically revolves around Russian-led election interference and communications between Trump officials and the Russian government during the campaign, it has evolved with recent developments. Mueller's team has expanded the probe to examine whether White House officials obstructed justice by attempting to improperly influence investigations into Flynn's conduct or whether Trump did so by firing Comey. Investigators have also begun looking into Trump's finances.

Many have seen Trump's eagerness to criticize and combat Mueller as evidence that he is intent on burying the Russia investigation by any means necessary. While legal experts are split on whether Trump could technically fire Mueller -- and what it might take to justify such an action -- many agree that it would invite a constitutional crisis and could even potentially serve as grounds for the president's impeachment.

But the investigation's significance is clearly more than symbolic. In July, FBI agents raided Manafort's home in Virginia, and seized “documents and other materials," the Washington Post reported in August. Manafort had retroactively registered as a foreign agent a month before the raid, following reports that his firm had been paid more than $17 million from 2012 to 2014 for lobbying work on behalf of a pro-Russian political party in Ukraine.