That probe is separate from another inquiry into former Illinois Gov. Rod Blagojevich's (D) attempt to sell President Obama's former Senate seat. Jackson has denied wrongdoing in that matter.

Jackson has been on an extended absence from Capitol Hill to undergo treatment for depression at the Mayo Clinic in Minnesota. 

The nine-term lawmaker left Washington in mid-June, revealing two months later that he was being treated for bipolar disorder. He returned to the capital briefly in September, but readmitted himself to the Mayo to continue his treatment. 

Jackson easily won reelection on Tuesday, despite not campaigning.

The allegations into misuse of campaign funds first surfaced in October, when The Wall Street Journal reported that the FBI and U.S. attorney’s office were spearheading the investigation.