The panel said it did not imply McMorris Rodgers had committed a violation, and did not explain the allegations. USA Today said the case apparently deals with allegations McMorris Rodgers improperly mingled congressional and campaign funds.
The case was referred to the Ethics Committee by the independent Office of Congressional Ethics after a preliminary review.
McMorris Rodgers chairs the House Republican Conference, and is the top-ranking woman in Republican leadership. She was tapped to give the GOP response to President Barack Obama's State of the Union address last week.
USA TODAY reported Wednesday McMorris Rodgers had spent nearly $70,000 on legal services at the end of last year from firms that specialize in congressional investigations.
The newspaper said that suggests investigators may have been looking at the case for months.
"As has become an unfortunate rite of passage for many members of Congress, the OCE regularly refers matters to the House Ethics Committee for further review," McMorris Rodgers' attorney Elliot Berke said in a statement. "Such reviews are virtually automatic, and as the committee always points out, does not indicate that any violation has occurred, or reflect any judgment on behalf of the committee.
"The congresswoman and her office cooperated fully with the OCE during its inquiry and have already begun assisting the committee with its review," Berke said.
The Ethics Committee also said Thursday it is looking into allegations against Rep. Markwayne Mullin, a first-term Republican from Oklahoma. His office was not immediately able to provide details of that case, USA Today said.