A federal grand jury in Springfield investigating former U.S. Rep. Aaron Schock, R-Ill., is exploring his relationships with his very best donors.

And one example of the way Schock used his deep Illinois donor base to raise money and expand his influence was a lavish fundraiser in central Illinois last Aug. 19, marketed as “Aaron Schock’s Summer Sit Down 2014.”

The event’s “special guest” was Sen. Marco Rubio, the Florida Republican who launched his presidential bid in April.

Rubio will be in Chicago on Tuesday for his first major Illinois fundraiser since announcing his White House run; it takes place at 11:30 a.m. at the Union League Club.

An issue for the federal authorities looking at Schock may be that there are no records of in-kind contributions for the Aug. 19 event – expenses unaccounted for in federal disclosure reports. All costs of fundraisers – whether paid directly by the political committee or donated — must be reported.

Last summer, Rubio, then pondering a presidential run, scheduled a fundraiser for his Reclaim America political action committee in Chicago. After that, he traveled to downstate Brimfield, a town near Peoria, to be the star draw at the Schock fundraiser.

After a Chicago fundraising event, U.S. Sen. Marco Rubio, R-Fla., traveled to downstate Brimfield to be the star draw at an Aaron Schock fundraiser.

After a Chicago fundraising event, U.S. Sen. Marco Rubio, R-Fla., traveled to downstate Brimfield to be the star draw at an Aaron Schock fundraiser.

Schock designed the event to raise money for four House Republicans and several political funds Schock controlled.

Days later, Rubio’s PAC reported receiving $20,000 in total from two couples with connections to Schock: Darren and Rebecca Frye of downstate Washington, and Brenda and Norman Johnson, from Peoria, where Schock lived. The Fryes and Norman Johnson were at the Aug. 19 event, a source told the Chicago Sun-Times.

Schock resigned from Congress on March 31, after questions were raised about his spending of government and political funds. Since then, Schock, once a GOP rising star, has kept a relatively low profile. He was spotted last week at Joe’s Stone Crab in Washington, D.C., huddling with his criminal defense attorney, George Terwilliger.

On Aug. 18, 2014 — the day before the Brimfield event — Schock created a new political committee and called it the Schock Majority Fund. According to Federal Election Commission records, the other House Republicans to benefit from the joint fund were Rep. Rodney Davis, of Illinois; Rep. Bill Flores, of Texas; Rep. Diane Black, of Tennessee, and Rep. Tom Graves, of Georgia.

In past years, before his downfall, Schock would organize an annual grand summer event, snaring big names, for example: former first lady Laura Bush; House Speaker John Boehner, of Ohio; and Rep. Paul Ryan.

Rubio was a big get for Schock in 2014.

It was also a reasonable political investment for Rubio to take the time to get to Brimfield. The March 2016 Republican primary in Illinois is relatively early. Schock’s best deep-pocketed donors would all be in one place and Schock’s conservative, solid GOP district was a good fit for Rubio.

Schock arranged for his summer bash at the home of Rita Kress, the chief of the Kress Corp., who is listed in FEC records as a Schock donor.

For federal bookkeeping purposes, there were two events at the Kress home that day. First was a party in a tent on her lawn with the proceeds to the Schock Victory Fund.

After that, there was a private multicourse dinner in the Kress home for the Majority Fund, featuring dishes from Topolobampo, the famous restaurant in Chicago.

A Sun-Times examination found no records to show payments or in-kind contributions for the dinner at the Kress home benefitting members of Congress, which by law have to be reported. There are no records of payment for the wine, chef expenses and other costs.

Schock did not reply to an email asking him for comment.

Darren and Rebecca Frye also donated $12,500 each to the Schock Majority Fund.

A story about the Fryes on Monday in the Chicago Tribune raises questions over a possible Schock-related donor “swapping” exchange to circumvent federal donation laws.

In 2010, the Fryes and other Schock donors contributed to the campaign of former Rep. Michael Grimm, R-N.Y., who pleaded guilty in 2014 to tax felony charges. Several Grimm donors then contributed to Schock.

The Tribune also reported that Schock “regularly traveled with help from a Frye company that operated a Cessna airplane.” Schock also stayed at condos owned by the Fryes on Chicago’s Near North Side. House members cannot accept free housing. If the stays were a gift, Schock failed to report them.

The Tribune also reported that the Fryes were among donors who went with Schock on a 2011 trip to Saudi Arabia. Schock was required to report the trip.

The Schock grand jury is expected to hear more from witnesses in June.