The Federal Trade Commission (FTC) has subpoenaed Apple as part of its antitrust investigation of Google.
The agency is seeking information about how Apple incorporates Google's search engine into its iPad and iPhone, sources told Bloomberg on Wednesday. The FTC's request for documents includes the contract that made Google the default search engine on Apple devices.
Microsoft, which operates the Bing search engine, has criticized Google's arrangement with Apple, calling it anticompetitive.
The late Apple founder Steve Jobs accused Google of copying his company's technology, and Apple has launched a patent war against Google and the companies that use its Android system.
Although the FTC has not publicly discussed the focus of its investigation, lawmakers have questioned whether Google is using its dominance in online search to unfairly boost its other services.
Google Chairman Eric Schmidt defended his company's business practices at a Senate antitrust hearing in September. He denied allegations that Google is artificially ranking its own services higher than its competitors'.
Despite the regulatory scrutiny, Google began highlighting content from its social networking site Google+ in search results in January. Critics argue the feature unfairly gives a preference to Google's own service over competitors like Facebook and Twitter.