U.S. federal regulators have begun a formal antitrust investigation into Google's business practices.
In a blog post Friday, the internet search giant said it received notification from the Federal Trade Commission of the review on Thursday.
Google said "it's still unclear exactly what the FTC's concerns are."
But the inquiry is expected to focus in large part on whether Google abuses its dominance of Internet search to extend its influence into other lucrative online markets, such as mapping, comparison shopping and travel. Rivals complain that Google, which handles two out of every three Internet searches in the U.S., manipulates its results to steer users to its own sites and services and bury links to competitors.
The European Commission and the Texas attorney general have already opened investigations into whether Google uses its enormous clout as a major gateway to the Internet to stifle competition online. The U.S. Senate Judiciary Committee's antitrust subcommittee, too, is examining whether Google gives its own services favorable treatment in search results.
Google maintains that most of the accusations of anticompetitive behavior come not from users, who like its services, but from competitors that are not pleased with their search rankings.
"Since the beginning, we have been guided by the idea that, if we focus on the user, all else will follow," the company said in its blog post.
"We make hundreds of changes to our algorithms every year to improve your search experience," it added. "Not every website can come out at the top of the page, or even appear on the first page of our search results."
Earlier, the Wall Street Journal and several other media outlets reported that the Federal Trade Commission was about to launch an investigation. They said the attorneys general in California, New York and Ohio have also launched their own antitrust probes. All reports cited unnamed sources "familiar" with the investigations.
In midday trading Friday, Google's stock fell $6.66 or 1.4 percent, to $473.56.