Lots of info in this June 27, 2018 letter from Rosenstein to Grassley. DOJ is not supposed to remark on investigations unless they indict. Comey and somebody at the DOJ broke that rule when Comey told the world that FBI had the Trump campaign under a counterintelligence (not criminal) investigation. Mueller is a result of breaking the rules.
https://www.justice.gov/ola/page/file/1080041/download