Republican Congressman Tom Petri requested the investigation in February. Petri has since said he will not run for re-election in November, retiring after 18 terms in Congress.
Gannett Wisconsin Media published articles drawing a link between companies that have business before Congress and stock Petri owns in those companies.
One article reported that Petri bought hundreds of thousands of dollars' worth of stock in Oshkosh Corp. and then pressed colleagues to help the company retain a $3 billion military contract and avoid Pentagon cuts.
Petri issued the following statement Monday: "I look forward to the Committee on Ethics completing its review. I remain confident that the committee will find that I acted properly, and that I reasonably sought, relied on, and followed the committee's advice and that I complied with House rules."
In a joint statement, the ethics committee's Republican chairman and ranking Democrat said an extension does not reflect any judgment by the committee. The committee leaders said they will announce their next course of action by Sept. 30.
The committee's jurisdiction over Petri ends when he leaves office.