The Federal Communications Commission (FCC) asked Apple and AT&T to explain the decision to reject an application developed by Google for the iPhone.
FCC chairman Julius Genachowski said "inquiry letters" were sent on Friday to Apple, AT&T, the exclusive carrier for the iPhone in the United States, and Google.
"The Federal Communications Commission has a mission to foster a competitive wireless marketplace, protect and empower consumers, and promote innovation and investment," Genachowski said in a statement.
He added that in sending the letters the FCC wanted to get "the facts and data necessary to make the best policy decisions on behalf of the American people."
The FCC move comes after Google said a Google Voice application for the iPhone was rejected by the iPhone App Store and related applications were removed.
In the letters to Apple and AT&T, the FCC asked whether the two companies had consulted in deciding to reject the applications, which have been described as a potential threat to services provided by the iPhone and AT&T.
Exclusive deals between mobile phone makers and wireless carriers have been drawing scrutiny from the FCC and the US Congress and President Barack Obama's antitrust chief, Christine Varney, has pledged to be more aggressive than the previous administration in cracking down on practices seen as anti-competitive.
Google Voice allows users to merge their home, office and mobile phones into a single number. It also allows them to make cheap international phone calls, send free SMS messages and provides transcripts of voicemail messages.
The FCC gave Apple, AT&T and Google until August 21 to respond to its inquiries.
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