WASHINGTON - The Justice Department is investigating the congressional expenses and business deals of Illinois Rep. Aaron Schock, and FBI agents have begun issuing subpoenas to witnesses, a person familiar with the case told the Associated Press on Friday.

Investigators were focusing on Schock's House office expense account, expenditures by his reelection campaign, and his personal investments with longtime political donors, the person said. Schock, 33, a young, media-savvy Republican, abruptly announced his resignation Tuesday after weeks of mounting media reports about questionable expenditures and personal finances.

The government was convening a federal grand jury in Springfield, Ill., according to the person, who was not authorized to publicly discuss the case. The person also said that FBI agents were visiting people close to the Republican congressman who were being compelled by subpoena to testify. The grand jury was hearing testimony in early April, according to the subpoenas.

A spokesman and lawyers for Schock did not respond to repeated phone calls and e-mails Friday from the Associated Press.

Schock's sudden resignation followed revelations over six weeks about his business deals and lavish spending on travel, personal mileage reimbursements, and office redecorating in the style of Downton Abbey.

Congressional ethics investigators had begun probing Schock's conduct in the days before his announcement, but that probe was expected to shut down because of the federal investigation.

Questions have included Associated Press investigations of Schock's real estate transactions, air travel, and entertainment expenses - including some events that Schock documented in photographs on his Instagram account.

On Monday, the AP confirmed that the Office of Congressional Ethics had reached out to Schock's associates as it apparently began an investigation.

The owner of an air charter service in Peoria confirmed Friday that he had been contacted by an ethics investigator interested in Schock's extensive flights on planes owned by campaign donors. Harrel W. Timmons, owner of Jet Air Inc., was not a Schock donor but said the investigator wanted to know about the lawmaker's flights on a plane owned by D&B Air, a Peoria aviation firm owned by a prominent Schock donor.

AP previously reported that Schock's use of the D&B plane appeared to violate congressional rules in place at the time prohibiting the use of office accounts to pay for private flights. Schock had used office expenses to pay $24,000 for eight flights in 2011 and 2012. Since mid-2011, Schock's office and campaign expenses paid for more than $40,000 worth of flights on planes owned by his political donors.