The Federal Trade Commission (FTC) has opened an antitrust probe into Google's recent acquisition of DoubleClick, according to The New York Times.
A source told the NYT that a preliminary investigation has been launched into the deal, in which Google paid $3.1bn for the ad company.
Microsoft, which was beaten to the acquisition by Google, expressed concerns that the deal makes the online ad market less competitive, while WPP's Martin Sorrell was concerned about giving Google access to clients' data.
Meanwhile, consumer privacy groups, led by the Electronic Frontier Foundation, have opposed the deal, amid concerns about the huge amount of personal data held by the two companies.
As privacy issues are not normally the focus of antitrust cases, it is believed that the investigation will look at whether the deal will give Google a monopoly over online advertising.
The FTC will decide whether to extend its investigation in the next few weeks, while Google says it is confident that the deal will stand up to scrutiny.
How does the Google-DoubleClick deal affect you?