A regulatory filing revealed few details except that the investigation involved how some unnamed advertisers used Google's advertising network, which generated more than $8 billiion during the first quarter.
Google's ad network, which primarily delivers short text ads alongside search results and other Web content, is the main way the company makes money.
The probe looks into whether Google unfairly manipulates its search results to favor its own services and rigs its ad system to drive up prices.
Google said it decided earlier this month to set aside $500 million to cover a possible settlement.
The Associated Press contributed to this report.