Federal agents were seeking evidence of child pornography when they searched the home and offices of Elgin state Rep. Keith Farnham last week, according to the Chicago Tribune.
Farnham, 66, resigned from his position on Wednesday, citing health issues, a day after winning the primary running unopposed.
It's not clear what, if anything, investigators found after executing the search warrant, the newspaper reported, but several computers were seized in a search conducted by Immigration and Customs Enforcement.
Federal officials say Farnham has not been accused of any wrongdoing.
Farnham told the Tribune he couldn't comment on the case.