WASHINGTON -- The Federal Communications Commission has launched an inquiry into why Apple Inc. rejected Google Inc.'s Internet-telephony software for the popular iPhone, another sign of the Obama administration's stepped-up scrutiny of competitive practices in the technology industry.

In letters sent late Friday to the two companies and AT&T Inc., the FCC asked why Apple rejected the Google Voice application for the iPhone and removed related applications from its App Store. The letter also seeks information on how AT&T,...