Milwaukee County Assistant District Attorney David Feiss says his office provided the FBI with information connected to his investigation into Senator Chuck Chvala.
That would include allegations that Chvala directed lobbyists to contribute to the national Democratic Legislative Campaign Committee.
Prosecutors allege the money came back to Wisconsin in large amounts to help Chvala win election for Democratic Senate candidates.
Feiss says those matters are beyond his jurisdiction.
Feiss charged Chvala last year with several felony counts, including extortion.
Milwaukee FBI spokeswoman Monica Shipley confirmed there's an investigation, which The Washington Post first reported in today's editions.
The Democratic Legislative Campaign Committee was established to promote Democratic control of state legislatures nationwide.