Google Inc said the Department of Justice has asked for more information regarding its acquisition of online advertising company AdMeld.
The DoJ request does not come as a surprise as today's display advertising industry is very new and highly complex, Google said on its Public Policy blog.
The company will work to enable this review to be concluded as quickly as possible, Google said.
On June 13, Google said it would buy AdMeld to grab a larger slice of the market for graphical display ads.
Pressure is mounting on Google as antitrust regulators have started taking a harder look at its acquisitions, such as last year's AdMob and travel software company ITA Software.
The Federal Trade Commission announced a probe into business practices at Google last month.
Google's shares closed at $607.22 on Wednesday on Nasdaq.
(Reporting by Divya Sharma in Bangalore)