The Justice Department is formally investigating US Representative Aaron Schock.
The Peoria Republican submitted his resignation Tuesday.
The federal government is investigating whether the 33-year-old committed crimes with his office expenditures and business dealings.
The source says the government is convening a federal grand jury in Springfield and the FBI has started issuing subpoenas to people close to the Republican congressman. The source spoke only on grounds of anonymity because the person wasn't authorized to publicly discuss the case.
Schock abruptly resigned Tuesday following a six-week cascade of revelations about his business deals and lavish spending on everything from overseas travel, to a Bears game, a Katy Perry Concert and office decor in the style of "Downton Abbey."