The investigation, which began before Jackson took medical leave from Congress on June 10, was initially focused on activity in the congressman's campaign fund, the Chicago Sun-Times reported.
The source said Thursday the investigation has since gone into other areas but did not elaborate, the newspaper reported.
Federal investigators are "going down every rat hole," the source said, adding that the FBI crew investigating "is not yet finished digging."
Attkisson leaves CBS News, reportedly over network's 'liberal bias'
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