The British competition agency CMA is also investigating Google's ad practices.
Last year, Google generated $147 billion in revenue from online ads, more than any other company in the world, with ads including search, YouTube and Gmail accounting for the bulk of its overall sales and profit.
The company's display or network business, in which other media companies use Google technology to sell ads on their website and apps, accounted for about 16% of its revenue.
(Reporting by Foo Yun Chee; Editing by Jan Harvey and David Evans)