The investigation is unrelated to alleged efforts by Jackson, D-Ill., to purchase the U.S. Senate seat vacated by President Barack Obama, a scandal that sent disgraced Illinois Gov. Rod Blagojevich to prison.
The Sun-Times said the investigation is focused on "suspicious activity" related to the operation of Jackson's District of Columbia office.
House members had an allowance of $1.4 million-$2 million to operate their congressional offices in 2010, the House website says.
Jackson, 47, the son of civil rights leader the Rev. Jesse Jackson, has been on leave from his House seat since mid-June due to intestinal problems and mental health issues. He was first elected to Congress in 1995.
Jackson has been absent from the House since June 10 and hasn't been campaigning for re-election although his name remains on the election ballot.