Nov. 21, 2012 -- Rep. Jesse Jackson Jr., the embattled Democrat from Illinois who has been on medical leave from Capitol Hill for months undergoing treatment for bipolar disorder, resigned from the House of Representatives Wednesday.

Aides to House Speaker John Boehner, R-Ohio, said they had received a letter of resignation from Jackson Jr.

The news was first reported by ABC News' Chicago station WLS and two Chicago newspapers.

"For seventeen years I have given 100 percent of my time, energy and life to public service. However, over the past several months, as my health has deteriorated, my ability to serve the constituents of my district has continued to diminish," the letter read. "Against the recommendations of my doctors, I had hoped and tried to return to Washington and continue working on the issues that matter most to the people of the Second District. I know now that will not be possible."

Jackson has faced a slew of problems in recent months, most recently a probe by federal investigators into his finances. The federal probe was trying to get to the bottom of "suspicious activity" connected to Jackson's House seat and potentially inappropriate expenditures.

There are no developments expected in the ongoing investigation and plea negotiations in the near term, according to federal officials familiar with the investigation. Jackson has been in talks with the Justice Department about a possible deal regarding allegations of campaign finance violations.

Part of the investigation's focus is on whether Jackson improperly used campaign funds for personal purchases, including furnishings for his Washington, D.C., home. Investigators are also looking at allegations that Jackson bought a female friend a $40,000 Rolex watch. The investigation is being run by the FBI's Washington Field Office.

Jackson's attorney, Dan Webb, said he is cooperating with the investigation.

"We hope to negotiate a fair resolution of the matter, but the process could take several months," Webb said in a statement to ABC News.

Jackson's problems began in June when the son of civil rights leader, Jesse Jackson, suddenly left Congress. His office said he was seeking treatment for "exhaustion."

Two weeks later, his office noted that his condition was "more serious" than initially thought.

Jackson, whose district includes a large portion of Chicago's South Side and southeast suburbs, then spent some time at a treatment facility in Arizona before later moving to the Mayo Clinic in Minnesota.

Finally, in August, the clinic said Jackson was being treated for bipolar disorder, "responding well" and "regaining his strength."

In early September, Jackson returned home to his family in the nation's capital. A source told ABC News that day that Jackson "sounded good." However, despite the congressional summer recess ending a week later, Jackson did not return to work.

Jackson has missed 230 votes in the House this year. He last voted on June 8.

Despite all his troubles, Jackson still managed to win re-election in a landslide earlier this month. But now, he will leave Congress shrouded by personal problems and professional probes.

Jackson's resignation could cost Illinois taxpayers $5.15 million, the state board told ABC News. Illinois will hold two special elections to replace Jackson, a primary and a general election. Looking at previous special House elections in recent years, the board found that each election could cost the state and relevant counties around $2,575,000 this year.

That's just a projection, and that's assuming the 2nd Congressional District holds its special primary and general elections as freestanding votes. Holding the special elections at the same time as other, minor elections could save some money.

However, Jackson may have cost taxpayers even more by resigning so suddenly, because the statutory timeline would likely allow for the primary, but not the general election, to be held on Illinois' consolidated election schedule.

Illinois primaries will happen Feb. 26, but the April 9 general-election date falls outside the 115-day special-election range. Counting Thursday as day 1 in such a timeline, March 21 would be the earliest the 2nd District special election could be held.

Holding a statewide special election to replace governor Rod Blagojevich cost the state between $90 million and $100 million, according to estimates, the state elections board said.

One Chicago-based Democratic strategist told ABC News he expects to see a real "food fight" for this heavily Democratic seat.

The list of possible Democratic candidates for the seat included state Sen. Napoleon Harris, a former NFL linebacker; Will Burns, alderman of the Fourth Ward; Robin Kelly, a former state representative and 2010 candidate for state treasurer; and state Sen. Toi Hutchinson.

House Minority Leader Nancy Pelosi, D-Calif., met with Jackson and his father today.

"It is with great sadness that we learned of Congressman Jackson's decision to submit his resignation. His service in Congress was marked by his eloquent advocacy for his constituents' views and interests," Pelosi said in a statement. "Through his public statements and his writings, he presented a fresh perspective on how we work together to form a more perfect union. As he works to address his health, our thoughts and prayers are with him, his wife Sandi, his children as well as his parents. We are grateful to him and his family for their longstanding record of public service to our country."

Reached by ABC News Wednesday, one of Jackson's colleagues, Rep. Danny Davis, D-Ill., said he was not surprised that Jackson had decided to leave Capitol Hill, noting that Jackson had cancelled a conference call with constituents set for Wednesday morning.

"I did get the feeling that, if you've got plans to say something this morning and then you don't, that would probably indicate that you were pretty close to something," Davis said.

ABC's John Santucci, Amy Walter, John Parkinson, Jason Ryan and Sarah Parnass contributed to this report.