They were appointed under different authorities, with radically different rules for reporting and disclosure. Starr was required by the independent counsel statute governing his appointment not only to conduct a criminal investigation but also to submit a report to Congress if he found any evidence of impeachable offenses.
However, that statute expired in 1999. Mueller was appointed under the far narrower Justice Department regulation for special counsels that replaced the independent counsel statute. Far from requiring a report to Congress of impeachable offenses, these regulations tightly circumscribe Mueller’s ability to do so.
Section 600.8(c) of the regulations provides that the special counsel shall provide the attorney general with a “confidential report explaining the prosecution or declination decisions reached by the Special Counsel.”