In a three-sentence news release, Jackson's office disclosed that the Democrat went on leave June 10 but did not provide further details, including how long he would be away. In the release, he asked that his family's privacy be respected.
"I don't know how long he'll be out of action," said Frank Watkins, Jackson's spokesman, adding that he could not offer additional details.
Several of Jackson's congressional colleagues noted his absence last week on the House floor, where he missed votes. U.S. Rep. Danny Davis, a Chicago Democrat, did not know why he was gone.
The statement said Jackson's offices will remain open for constituent services. Since June 10, the congressman's office has sent out news releases quoting him. His Twitter site has remained active, including two tweets Monday about the Affordable Care Act, which the U.S. Supreme Court is expected to rule on this week.
Earlier this year, Jackson, 47, faced a primary challenge from former U.S. Rep. Debbie Halvorson and campaigned more actively than in previous years. Since then, the veteran Democrat has been promoting a third Chicago area airport and campaigned against a proposed federal detention center for illegal immigrants.
Jackson first won office in 1995 and represents Illinois' 2nd District, which includes neighborhoods on Chicago's South Side and in its south suburbs.
The House Ethics Committee probe involves allegations that Jackson was involved in discussions about raising money for then-Illinois Gov. Rod Blagojevich's campaign in exchange for Blagojevich appointing him to President Barack Obama's vacated U.S. Senate seat.
Jackson also allegedly directed a fundraiser and longtime friend, Raghuveer Nayak, to buy plane tickets for a woman described as Jackson's "social acquaintance." Jackson has since called it a personal matter that he has dealt with privately with his wife, Chicago Alderman Sandi Jackson.
Last week, Nayak was arrested and pleaded not guilty to unrelated fraud charges involving outpatient surgery centers he owns. At Blagojevich's first corruption trial in 2010, prosecutors said another Blagojevich fundraiser was ready to testify that Jackson instructed Nayak to raise money for Blagojevich's campaign in exchange for the Senate seat. The same witness later testified that he attended a meeting with Jackson and Nayak.
Jackson has not been charged. He has repeatedly denied wrongdoing and insists he will be vindicated.
Jackson's father, the Rev. Jesse Jackson, was out of the country Monday and declined to comment on the matter. Messages left for Sandi Jackson were not immediately returned.
Copyright 2012 The Associated Press.