The US Federal Trade Commission, which began an antitrust investigation into Google a month-and-a-half ago, is looking specifically at Google's relationship with Android handset makers, according to reports.
It is also looking at whether Google favours its own services in search results, the Wall Street Journal has reported.
Google said in late June that the FTC had initiated a review of its business, but it said it did not know exactly what concerns the FTC had.
Citing people familiar with the investigation, the Wall Street Journal said on Wednesday that the FTC is looking into the deals that Google makes with smartphone makers that use its Android phone software.
Observers said that this appears to be related to allegations made by Skyhook Wireless, which has sued Google, alleging that handset makers must use Google's location services if they want to include the Android Market on their phones.
The FTC is also investigating whether Google is taking information like local business reviews that is collected by competitive sites for use on its own sites, the Journal reported. The agency also wants to know if Google is giving its own sites, such as Places pages and Google Finance, higher placement in search results than competitive sites.
Google did not immediately reply to a request for comment about the report.
Google is also facing a separate antitrust investigation launched last year by the European Commission. That investigation is focused on a similar issue -- whether Google abused its dominant position in search to promote its own services.