This is a significant step, though it is my understanding that by assigning someone from Connecticut, it means that they are not technically a special prosecutor.
Obviously the fact that a criminal probe has been opened is significant, and the man assigned, John H. Durham, was the one who successfully investigated and prosecuted the FBI agents who protected James “Whitey” Bulger and Stephen “The Rifleman” Flemmi because they used them as informants.
However, I do not expect him to be given much in the way of freedom over his investigation.