Washington (CNN) The FBI and the federal prosecutors in Illinois are investigating whether Rep. Aaron Schock broke the law in accounting for campaign expenses, according to people familiar with the matter. 
 
 Schock in recent days announced plans to resign, citing controversy surrounding allegations that he improperly accounted for travel and other contributions from donors and mileage reimbursements for a personal car he purchased with campaign funds. But the probe managed by prosecutors in the U.S. Attorney's office in Springfield, Ill., means his legal troubles are just beginning. 
 
 The federal investigation is focused on at least three questions, according to people familiar with the inquiry, including the mileage reimbursements, travel that was paid for by donors and in-kind contributions from donors that were not reported. 
 
 
 
 The FBI is delivering subpoenas in Washington and Illinois, according to two people familiar with the investigation. 
 
 
 
 Investigators from the U.S. Postal Inspection Service and IRS are pursuing various parts of the Schock investigation, according to law enforcement sources. 
 
 
 
 
 
 A U.S. law enforcement official says the investigation is at an early stage. Justice Department officials were monitoring published reports about Schock's expense troubles and were taken by surprise when he announced his resignation, according to another U.S. law enforcement official. A controversy that began with questions sparked by a light-hearted profile of Schock's "Downton Abbey"- themed office ballooned into a full-blown scandal over the past month that caused the rising GOP star's rapid fall from grace.A U.S. law enforcement official says the investigation is at an early stage. Justice Department officials were monitoring published reports about Schock's expense troubles and were taken by surprise when he announced his resignation, according to another U.S. law enforcement official. 

 WASHINGTON (AP) — A person familiar with the case tells The Associated Press that the Justice Department is formally investigating whether Rep. Aaron Schock of Illinois, who has submitted his resignation, committed crimes with his office expenditures and business dealings. 
 
 The government is convening a federal grand jury in Springfield, Illinois, this source says, and the FBI has started issuing subpoenas to compel people close to the Republican congressman to testify. The source spoke only on grounds of anonymity because the person wasn't authorized to publicly discuss the case. 
 
 Schock abruptly resigned earlier this week following a monthlong cascade of revelations about his business deals and lavish spending on everything from overseas travel to office decor in the style of "Downton Abbey." 

 AP Photo FBI begins probe of Aaron Schock spending 
 
 The Federal Bureau of Investigation has begun interviewing people close to Illinois Rep. Aaron Schock, signs of a criminal investigation and serious legal trouble for the Republican congressman, according to sources familiar with the probe. 
 
 Schock, a 33-year-old Republican, announced he would resign at the end of the month after allegations of misspending campaign and political funds. 
 
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 A federal grand jury in Illinois has been hearing evidence related to this case and is scheduled to receive testimony from witnesses next month, according to sources familiar with the issue. 
 
 Until this point, there had been no indication that the FBI or Justice Department had been involved in the scandal that led to Schock’s political implosion. The Office of Congressional Ethics had begun to look into the matter, but with his resignation, any congressional inquiry would end. 
 
 However, with federal investigators now getting involved, Schock’s status as a lawmaker would have no impact. A criminal probe could continue long after he left office. 
 
 A spokesman for Schock had no comment. 
 
 Schock stunned his colleagues and House GOP leaders on Tuesday when he announced his resignation. This came shortly after POLITICO asked him to respond to questions over tens of thousands of dollars in mileage reimbursement payments for use of his personal car from the federal government and political committees. 
 
 POLITICO, the Chicago Sun-Times and other news organizations have also reported on potential improper gifts to Schock, as well as official and political committee payments for use of chartered aircraft on personal trips.
– Rep. Aaron Schock may have thought he could simply disappear into the Illinois sunset after resigning his House seat on Tuesday, but it looks like his troubles are far from over. A source familiar with the case but speaking on condition of anonymity tells the AP that the Justice Department has started a formal criminal investigation into the 33-year-old rep's allegedly illegal campaign expenses, adding that the FBI has been issuing subpoenas to those close to the Republican. A grand jury is being convened in Springfield, Ill. Per CNN, the investigation will look at whether Schock improperly accounted for spending on everything from travel to a Downton Abbey-decorated office. The jury will reportedly start listening to testimony next month. The Office of Congressional Ethics had started poking around the Schock case, but once he resigned, that scrutiny would likely end, Politico points out. Not true of a criminal investigation—those leading it wouldn't care if Schock is a current rep or not, the site notes.