By Katie Marsal

Published: 08:40 PM EST (05:40 PM PST)

Apple's decision to reject Google Voice apps for the iPhone -- possibly at AT&T's request -- has prompted an FCC investigation into the anti-competitive nature of the move. A day later, AT&T has briefly denied the implied accusations.

The US government agency on Friday sent letters to Apple, AT&T and Google asking them to explain their roles in both rejecting Google's own app as well as pulling at least two third-party apps that were already available.

Besides inquiring into Apple's involvement, the letters also ask AT&T whether it was asked for an opinion and Google to outline both Google Voice as well as whether Google has had other apps approved. In practice, Google has only released a handful of apps but has ported over Google Earth from the desktop and used once-hidden programming instructions from Apple to develop the voice search component of Google Mobile App for the device.

The questions come as part of a larger investigation into the access to exclusive phones for rural customers, some of whom can't buy an iPhone or a similar handset simply because the relevant carriers don't operate in their areas.

What if anything the FCC suspects isn't immediately apparent. However, it's probable that the inquiry will look into whether or not AT&T wanted Google Voice absent to prevent competition with its own services, as it doesn't significantly tax the carrier's data network but does render it much less expensive to call long distance numbers and send text messages.

On Saturday, AT&T indicated that it was aware of the implications but directly denied any involvement in the App Store approval process.

"AT&T does not manage or approve applications for the App Store," company spokesman Brad Mays said. "We have received the letter and will, of course, respond to it."

Neither Apple nor Google have commented on the investigation themselves, but AT&T in the past has freely acknowledged that it doesn't want voice over IP apps like Skype, or TV-to-phone streaming apps like SlingPlayer Mobile, running on iPhones using its 3G network due to bandwidth concerns.