The U.S. Department of Justice has filed a lawsuit to block AT&T from acquiring mobile competitor T-Mobile USA, with the agency saying the $39 billion deal would significantly reduce competition, jack up prices, and stifle innovation. The DOJ lawsuit, filed Wednesday in U.S. District Court for the District of Columbia, points to T-Mobile USA's role as a low-cost competitor to the three other nationwide mobile carriers as a reason to block the deal.