The probe will examine the company's practices in search and other businesses, The Wall Street Journal reported, citing unnamed people familiar with the matter.
Google was fined $1.7 billion earlier this year by European regulators for antitrust violations in the online advertising market.
It faces two other multi-billion dollar fines from European regulatory agencies for abusing its control of the Android operating system and for favoring its shopping service over others. All the penalties have been appealed.
A Federal Trade Commission probe of Google ended in 2013, with no action against the company.
Shares of Alphabet fell $14.91 to end at $1,106.50 Friday.