The U.S. antitrust investigation by the Federal Trade Commission into Google's dominant position in the internet search market has subpoenaed Cupertino-based Apple in a bid to find out more information regarding the search deal between the two companies, according to Bloomberg.

It is believed other unnamed mobile vendors and wireless providers have also received similar requests as part of the investigations started by the FTC last year. The goal is to determine whether the search giant is unfairly increasing advertising rates for competitors, and whether Google is showing favoritism towards its own businesses.

Google has been the search engine supplier for Apple's iPhone since 2007, and while Apple did recently consider the search services offered by Microsoft's Bing in 2010, they ultimately remained with Google. Google Maps is also Apple's favored mapping software for their mobile devices.

The FTC is now requesting that documents, including the contracts between the two companies be handed over so they can view the details of the agreements for Google's mapping and their search engine on Apple's mobile services.

This is in response to claims by software giant Microsoft, among others, that the Google's contractual agreements are anticompetitive. Currently, Google enjoys as 75-percent market share with Android and iOS combined in the US. In Europe, the market share is said to be closer to 90-percent. In comparison, the current US desktop search market share for Google is around 66.4-percent, according to the Guardian.

It is possible that the contract signed between the two firms may show whether the search giant is abusing its dominance of internet search in order to boost its earnings from the phone advertising market. Currently, it is understood that the two companies share the revenue earned from Google searches on iPhone, iPad and iPod Touch devices.

A recent report by Macquarie Capital stated that Google earned $1.3 billion in search-related revenue on Apple's mobile products in 2011. Of that income, Google kept $335 million, and handed Apple the remaining $1 billion in earnings.

In Europe, Joaquin Almunia, the antitrust chief of the European Commission stated on March 5 that a decision will be made next month on the future of their 18-month long probe into Google, and they will notify the company if they have any concerns.

Apple and Google have not responded to requests for further information.