The $4.5 billion Nortel patent sale to Microsoft, Research in Motion and Apple is reportedly the focus of a deeper investigation by U.S. antitrust regulators. The Wall Street Journal reports that the Justice Department is trying to determine whether the purchase would unfairly hurt smartphone makers that use Google's Android operating system.

The consortium of businesses out-bid Google for the patent portfolio auctioned off by the bankrupt Nortel Networks last month. The extensive patent portfolio -- 6,000 in total -- touches nearly every aspect of telecommunications and additional markets as well, including Internet search and social networking.

Companies licensing the Android operating system have faced an increasing number of patent-infringement lawsuits in recent years, especially from companies such as Microsoft and Apple. We've reported here about the increasing number of device makers entering patent-licensing agreements with Microsoft in the last year. According to reports, Microsoft is making more in these Android licensing deals than from licenses of its own Windows Phone.

The concern is that the Nortel patent sale could make Google an antitrust victim (though it did pick up over 1,000 patents from IBM last week).

The sale was finalized last week after receiving approval from U.S. and Canadian judges.