Sprint Nextel Corp. joined with the U.S. Justice Department, filing its own lawsuit aimed at blocking AT&T Inc.'s proposed $39 billion takeover of wireless carrier T-Mobile USA, a potential hurdle to AT&T's efforts to reach a settlement with the government or win a court battle to complete the deal.

By filing the suit, Sprint has the opportunity to make additional arguments that the government may not make," said Melissa Maxman, an antitrust attorney with law firm Cozen O'Connor. "They can argue more vigorously that they are...