WASHINGTON (AP) — 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 re-election campaign and his personal investments with long-time 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, Illinois, 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 emails Friday from AP.

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.

House ethics investigators typically stand down open inquiries once federal authorities open their own probe or when the House Ethics Committee orders a halt in the inquiry. The OCE had been authorized to continue its inquiry until Schock's planned March 31 resignation. His decision to quit has no impact on the FBI investigation.

Earlier this week, Schock's father, Richard, told an ABC reporter: "Two years from now he'll be successful, if he's not in jail."

"If you're going to investigate his real estate dealings, etc., then find out the facts," Richard Schock said. "The facts are what are going to convict him or exonerate him."

The AP reported last week that much of Schock's personal wealth, estimated at about $1.4 million, grew from a series of real estate deals involving other long-time political donors. Schock's political contributors built, financed and later purchased a house the lawmaker owned as an investment in Peoria. He owns a stake in a Peoria apartment complex involving other contributors. And he pushed for a federal appropriation that would have benefitted a donor's development project.

Schock's expenses came under scrutiny last month after the Washington Post reported that Schock had paid $40,000 from his House expense account for a lavish office redecoration modeled on decor depicted in the TV serial "Downton Abbey." Reports by Politico and other news organizations also singled out Schock's unusually high, personal reimbursements for auto mileage.

Schock responded to the growing scrutiny by paying back his office decorator $40,000. The day of his resignation, Schock also paid back his mileage expenses, but his spokesman did not say how much he had repaid.

In resigning abruptly on Tuesday, Schock cited a "heavy heart," following six weeks of revelations about his business deals. He said in a statement that the constant questions about his spending and business dealings had made it impossible to serve effectively as congressman.

House Speaker John Boehner was not informed of Schock's resignation before it was announced but has said he supported the decision to quit.

A spokeswoman for the Federal Elections Committee also confirmed Friday that staff lawyers were reviewing a complaint from a liberal-leaning watchdog group, Citizens for Responsibility and Ethics in Washington. The group complained Feb. 26 that two political committees associated with Schock paid more than $9,000 for flights on donor planes. The FEC does not investigate complaints until the full committee votes on the matter. That has not happened, the spokeswoman said.

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Associated Press writer Kerry Lester contributed to this report from Peoria, Ill.