From the Chicago Sun-Times:

The investigation — based in the Washington, D.C., FBI field office — is not related to the attempted sale of the U.S. Senate seat that figured in former Gov. Rod Blagojevich’s corruption conviction, but is focusing on “suspicious activity” involving the congressman’s finances related to his House seat and the possibility of inappropriate expenditures, sources said.

The probe was active in the weeks prior to Jackson taking a leave from his U.S. House post on June 10, a leave his office ultimately attributed to his need for treatment for a bipolar disorder, the sources said.

It was unclear whether the probe involved the congressman’s official House spending account or his campaign finance account. But one source said it was an account monitored by Congress.