At least four staffers who worked for former Illinois Rep. Aaron Schock have told Congress they've been subpoenaed to testify before a federal grand jury.

The House clerk reports subpoenas were issued to Schock deputy chief of staff Mark Roman, Peoria district office manager Bryan Rudolph, district chief of staff Dayne LaHood and executive assistant Sarah Rogers.

The subpoenas are part of an ongoing Justice Department probe into Schock's office and campaign expenses. A Springfield, Illinois, grand jury began hearing testimony about the matter earlier this month.

Schock, a Republican, resigned from Congress March 31 following months of press reports into his expenses. Those included improper mileage reimbursements and trips on his donors' aircraft.

Congressional staffers are required under House rules to notify the speaker if they've been subpoenaed.