An investigation into Google and its dominant position in the search market has subpoenaed Apple, requesting the computer giant to explain the use of Google's maps and search results in iOS.
The American antitrust investigation into Google by the Federal Trade Commission (FTC) is seeking to find out more about the deal which sees Google provide Apple with internet search results and mapping services for the iPhone, iPad and iPod touch.
US authorities have demanded the Cupertino company to hand over documents explaining the agreements between itself and Google, and according to Bloomberg other smartphone makers - as well as mobile networks - have also been subpoenaed.
Combining iOS and Android devices, Google commands a 75 percent share of the search market on mobile devices and as a result Microsoft - whose Bing search engine has a much smaller share - has called the agreements between Google and Apple anti-competitive.
The FTC investigation has made requests for certain documents fomr both companies, including the agreements that made Google the preferred search engine on Apple's iOS devices.
Originally the probe was to focus on whether Google is "abusing its dominance of internet search to boost revenue," but now includes other companies such as Apple.
Apple has used Google as the default search engine on iOS since the original iPhone was launched in 2007 and Apple also uses Google Maps extensively in iOS. But recent speculation surrounding different maps in the new iPhoto app has cast doubt over the companies' future relationship.