Charles Rex Arbogast / AP Rep. Jesse Jackson Jr. in 2011.

Federal prosecutors and FBI agents in Washington have launched a new criminal investigation of Illinois Rep. Jesse Jackson, Jr. involving alleged financial improprieties, including possible misuse of funds monitored by Congress, law enforcement sources tell NBC News.

The probe prompted lawyers for Jackson — who has been on a leave of absence from Congress since June for medical treatment — to meet with federal prosecutors this week in an attempt to persuade them not to bring charges against the congressman, sources said.

The sources said it was unclear whether Jackson, who has not been seen in his office for months, would be charged before the November election — a subject that was discussed between Jackson’s lawyers and the prosecutors this week. Jackson’s lawyers urged the prosecutors not to file charges before the election — but prosecutors refused to make any commitments, the sources familiar with the meeting said.

Either way, the new investigation could ratchet up pressure for Jackson to step aside. Despite his illness — which his office has said involves his treatment for bipolar disorder — Jackson is running for re-election, seeking a 10th term. His lawyers did not return email and phone call request for comment.

Frank Watkins, Jackson’s congressional spokesman, says he has not reached out Jackson and has not spoken to him about the investigation, and that the first he heard of the investigation was when he was contacted by the Chicago Sun Times, which first reported the story. He said he believes Jackson is still in DC.

View NBCChicago.com's complete coverage of Jackson investigation

The sources, confirming the account in the Sun-Times, said the new probe is being run out of the U.S. attorney's office in Washington DC. They said it is unrelated to previous allegations that Jackson was part of a scheme to persuade ex-Illinois Gov. Rod Blagojevich to name him to the Illinois Senate seat of Barack Obama in exchange for $1 million in campaign contributions from a top fundraiser.

The sources did not specify the financial irregularities being investigated. But the Sun-Times said the case involves misuse of funds or an account monitored by Congress. It comes weeks after a report that Jackson and his wife, Chicago alderwoman Sandi Jackson, put their Washington DC home on the market for $2.5 million. A campaign spokesman said at first the home was put on the market to pay for medical bills, but the Jackson later took it off the market.

Jackson, the son of civil rights leader Jesse Jackson Sr., stopped working June 10, his staffers revealed two weeks later. He first obtained treatment at a facility in Arizona before transferring to the famed Mayo Clinic in Rochester, Minn., where doctors said he had "depression and gastrointestinal issues."

He left the Mayo Clinic and went back to Washington, D.C. in early September but has not returned to work.

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