Apple, in particular, has come under recent criticism for requiring software developers to use its programming tools to create applications for the iPhone and iPad. U.S. regulators plan to examine whether Apple's restrictions violate antitrust laws. The tensions between Google and Apple have been growing as they compete to sell mobile devices, services and advertising. The duel has fractured a once-cordial relationship that gained Schmidt a seat on Apple's board for three years. Schmidt resigned last summer as it became clearer that Google and Apple were on a collision course. Although more iPhones are in the hands of consumers, Android devices are catching up. Android powered 28 percent of the smart phones shipped in the U.S. during the first quarter, trailing only the BlackBerry at 36 percent, according to researcher NPD Group. The iPhone ranked third at 21 percent. In an attempt to make more money on mobile phones, Google is trying to buy mobile ad service AdMob for $750 million.