Four staffers of former U.S. congressman Aaron Schock have been subpoenaed to testify before a grand jury about his campaign spending.

The U.S. Department of Justice has been investigating the former congressman's expense accounts, relationship with campaign donors and re-election campaign spending over the last several weeks, according to the Associated Press. The ex-congressman's staffers have already been subpoenaed and began testifying at the beginning of the month.

The most recent Schock staffers to be subpoenaed are chief of staff Mark Roman, Peoria district office manager Bryan Rudolph, district chief of staff Dayne LaHood and executive assistant Sarah Rogers,  the news service reports. 

The special primary election to re-election Schock is scheduled for July 7. The general election will take place in September.