The two-year investigation by Robert Mueller and his team of investigators is rumored to be nearly complete. There are concerns, especially by House Democratic congressional investigators, that the details of that investigation will remain secret. They are publicly expressing hope that the full report details will be made public—or at least transferred to their respective committees of jurisdiction as they pursue their own investigations of President Trump, his family members, and others.
The Mueller probe was initiated to investigate the possibility that Russian government agents illegally influenced the 2016 presidential election. Supporters of the probe point to the indictments or convictions of 30 or more individuals so far, while Trump supporters note that, to date, none of those legal actions concern conspiracy or other crimes involving the 2016 election directly. (It should be noted that “collusion” is not a crime per se, but the term could be used to describe potentially illegal conduct involving a conspiracy.)
Beginning with the Watergate period in the 1970s, the US government has used three methods to investigate potential corruption and crimes at the highest levels. The first was a special prosecutor, the second was an independent counsel (think Kenneth Starr), and the third version is the special counsel statute currently in place. Wikipedia has an article discussing the history of the special prosecutor proc