The House’s franking commission is investigating whether Rep. Peter Roskam (R-Ill.) violated franking rules, after the nonprofit group Public Citizen filed a complaint Thursday.

Members are prohibited from sending out official, unsolicited mail 90 days before an election. But Public Citizen alleges that Roskam sent out several brochures to his constituents after the Aug. 6 cutoff.

The franking commission, headed by Rep. Mike Capuano (D-Mass.), received the complaint and it is “under review,” said Capuano spokeswoman Alison Mills.

Roskam’s office didn’t immediately return calls for comment Thursday, but a spokeswoman for the Republicans on the House Administration Committee — which oversees the franking commission — said Public Citizen will have to prove that Roskam sent out more than 500 pieces of official mail. If it’s less than 500, the mail would not reach the threshold for a “mass mailing” and thus not fall under franking rules, other than for content.

It’s the first complaint the franking commission has received for this election cycle, said House Administration spokesman Kyle Anderson. If the commission finds Roskam guilty of violating House rules, the issue would go to the Committee on Standards of Official Conduct.

In its complaint, Public Citizen alleges that Roskam sent “an undetermined quantity of mass mailings, including several distinct two-page color leaflets, praising the tenure and official work of Congressman Peter Roskam.”

“Right up into October, just weeks before the election, voters have been receiving two-page color mailers paid for by taxpayers, with Peter Roskam’s name splashed all over them,” Joan Claybrook, president of Public Citizen, said in a news release. “Taxpayers should not be footing the bill to advertise Roskam’s candidacy for Congress.”

A first-term Member, Roskam won by fewer than 5,000 votes in 2006. For this election, however, he has raised significantly more money than his Democrat opponent, Iraq War veteran Jill Morgenthaler, and is favored to hold his suburban Chicago seat.