Sprint Nextel sued to stop AT&T $39 billion purchase of T-Mobile USA, staking out its own private antitrust claims alongside the U.S. government's challenge to the deal.

Sprint may have filed its own case in the event that the Justice Department comes to a settlement with AT&T, said Eleanor Fox, a professor at New York University School of Law.

Sprint, the third largest U.S. wireless carrier, filed its lawsuit in the same federal court that is to hear the U.S. Department of Justice's (DOJ) case opposing the buyout. A fierce opponent of the deal, Sprint said the combination would lead to higher prices for consumers and create a duopoly between AT&T and Verizon Communications.