CHICAGO (AP) — Rep. Jesse Jackson, Jr., a once rising political star who has been on a mysterious medical leave for months for treatment of bipolar disorder, has resigned from Congress, a spokesman for House Speaker John Boehner said Wednesday.

Jackson's resignation and hushed absence from politics comes amid a continuing House Ethics Committee investigation into his dealings with imprisoned ex-Illinois Gov. Rod Blagojevich, along with reports of a new federal probe into possible misuse of campaign money.

Boehner's spokesman Michael Steele said his office received a resignation letter from Jackson but did not comment further.

Jackson, 47, disappeared in June, and it was later revealed that he was being treated at the Mayo Clinic for bipolar disorder and gastrointestinal issues. He returned to his Washington home in September, but went back to the clinic the next month, with his father saying his son had not yet "regained his balance." He left the clinic a second time earlier this month.

His return to the clinic in October came amid reports that he faced a new federal investigation into potential misuse of campaign funds. The Chicago Sun-Times first reported the probe, citing anonymous sources. An FBI spokesman in Washington, Andrew Ames, has told The Associated Press he could neither confirm nor deny the existence of a federal investigation into Jackson.

Jackson was easily re-elected on Nov. 6 representing his heavily-Democratic district, even though his only communication with voters was a robocall asking them for patience. He spent election night at the Mayo Clinic, but later issued a statement thanking his supporters and saying he was waiting for his doctors' OK before he could "continue to be the progressive fighter" they'd known for years.

Jackson, whose father is the Rev. Jesse Jackson, took office in 1995 after winning a special election in a landslide. Voters in the district have said Jackson's family name and attention to local issues have been the reasons for their support. He's easily won every election since taking office and brought home close to $1 billion in federal money for his district during his tenure.

He began his career in Washington with a star power that set him apart from his hundreds of House colleagues. But his resignation ends a once-promising political career that was tarnished by unproven allegations that he was involved in discussions about raising campaign funds for imprisoned former Illinois Gov. Rod Blagojevich in exchange for an appointment to President Barack Obama's vacated U.S. Senate seat.

The House Ethics Committee is investigating reports of those allegations, which Jackson has denied. After the allegations surfaced, he cut back drastically on his number of public appearances and interviews. Blagojevich is now imprisoned on corruption charges that accused him of trying to sell the seat, among other things.

The timing of Jackson's leave and the way it was handled also has invited scrutiny. Jackson's leave was announced just after a former fundraiser connected to the Blagojevich allegations was arrested on unrelated medical fraud charges.

Illinois Gov. Pat Quinn, a Democrat, has five days to schedule an election to replace Jackson after he receives official notice, and the election must be held within 115 days, according to election officials.

The vacancy left by Jackson's departure creates a rare opportunity for someone else to represent his district, which is made up of South Side Chicago neighborhoods, several southern suburbs and some rural areas. Even this year, when Jackson was absent during the crucial final months of campaigning, he easily defeated two challengers on the ballot, Republican college professor Brian Woodworth and Independent postal worker Marcus Lewis.

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Jackson reported from Washington.