Earlier today, the U.S. Department of Justice filed a complaint in U.S. District Court in Washington, D.C. seeking to block the $39B AT&T / T-Mobile merger. DOJ claims that the deal would violate U.S. antitrust law by removing a key competitor while giving AT&T more than a 30% share in the nationwide wireless market and more than a 50% share in many large metropolitan markets. DOJ's press release can be found here.
While certainly a twist in the road, this may not be the end of it for AT&T and T-Mobile. It remains to be seen whether AT&T can sell-off enough assets (likely a tall order, given that the complaint asserts that the relevant geographic market is the entire U.S.) and make enough commitments to settle-out of the complaint and get its deal done in a way that still makes sense for its shareholders.