The Federal Trade Commission is looking into the ties between the boards of Apple and Google, according to reports.
The New York Times said the inquiry involving Google and Apple centres on a possible breach of anti-trust laws.
Google boss Eric Schmidt and former Genentech boss Arthur Levinson sit on the boards of both companies.
The Clayton Anti-trust Act of 1941 forbids a person to be on a board of two rival firms at once if it reduces competition between them.
Apple and Google both offer competing web browsers and phone operating systems. However they have worked together, most recently to design versions of some of Google's services like Gmail and Google Maps for the iPhone.