A Chicago grand jury is expected to hand up an indictment Thursday afternoon against former Illinois Gov. Rod Blagojevich, sources told FOX News. 

Faced with a looming deadline to indict Blagojevich, federal prosecutors in Chicago said only that a court filing about a "significant criminal matter" will be issued Thursday. 

The U.S. Attorney's office said it plans to distribute a press release and court filing later Thursday. It did not say which case the filing would  address. 

But Justice Department sources told FOX News the "significant" matter is likely a Blagojevich indictment. The grand jury looking into the ousted governor's public corruption case meets only on Thursday, and the deadline for the U.S. attorney's office to bring an indictment against him is next Tuesday -- making Thursday the last day that the grand jury can take action.

Blagojevich was arrested on Dec. 9. Prosecutor Patrick Fitzgerald asked Chief Judge James F. Holderman of U.S. District Court for an extension to the Jan. 9 deadline for an indictment, saying he had to comb through the mounds of material to be presented to the grand jury. Fitzgerald could seek another extension on Thursday.

Blagojevich is accused of scheming to sell or trade President Obama's former U.S. Senate seat. His arrest ended his job as governor. The Illinois House impeached him Jan. 9. The Senate convicted him and removed him from office Jan. 29.

Blagojevich's publicist says he's not in Chicago. The family is apparently at Walt Disney World in Florida. 

FOX News' Mike Levine and The Associated Press contributed to this report.