The Justice Department is investigating Congressman Aaron Schock (R-IL).
The federal government is trying to determine whether Schock committed crimes with his office expenditures and business dealings.
The FBI has started issuing subpoenas to people close to him.
The next federal grand jury convenes in Springfield April 7.
Schock abruptly resigned Tuesday after a six-week cascade of revelations about his business deals and lavish spending on everything from overseas travel, to a Bears game, and "Downton Abbey" style office redecorating.
Schock is a couple months into his fourth term. His final day in office is March 31. After that, Governor Bruce Rauner has five days to schedule a special election to replace Schock. Rauner could set a date for a primary and a general election. The general election must be held within four months of Schock leaving office -- in this case, by the end of July.
Schock was scheduled to speak Saturday, March 21, at the McDonough County Republican Party's annual Lincoln Day Dinner but he has canceled that appearance.