Sources with knowledge of the probe told the Chicago Sun-Times that investigators believe Jackson had learned of the federal scrutiny of his financial activity prior to his June 10 leave from Congress.

The sources said it didn’t necessarily mean the tip was from an investigative source, saying it was possible the congressman received a tip from someone who was notified about the probe, possibly through a subpoena.

While Jackson, a South Shore Democrat, eventually released information concerning his health, he did not publicly disclose he had knowledge of the federal probe until last Wednesday — the day he submitted a resignation letter to U.S. House Speaker John Boehner.