The Chicago Sun-Times reported Wednesday that sources close to the investigation believe Jackson learned about the Justice Department action before taking medical leave from Congress in early June.
Jackson resigned from Congress last week, submitting a letter of resignation to Speaker John Boehner (R-Ohio) citing his health issues.
Jackson had been absent from Capitol Hill since last June, seeking treatment for severe bipolar disorder. The former lawmaker had not acknowledged the reported federal probe until his resignation.
"I am aware of the ongoing federal investigation into my activities and am doing my best to address the situation responsibly, cooperate with the investigators and accept responsibility for my mistakes, for they are my mistakes and mine alone," Jackson wrote in his letter.
“None of us is immune from our share of shortcomings or human frailties and I pray that I will be remembered for what I did right. It has been a profound honor to serve the constituents of Illinois’s 2nd congressional district,” Jackson added.
Reports before Jackson left office claimed his attorneys were attempting to negotiate a plea deal that would require him to resign from Congress and repay the campaign funds.
Jackson’s office has not commented on the reports of plea talks.
Former Rep. Debbie Halvorson (D-Ill.) is expected to run for Jackson’s seat in an upcoming special election set for Feb. 26.