Seven state attorneys general joined the Justice Department's lawsuit to block AT&T's proposed $39 billion acquisition of T-Mobile Friday.
The Justice Department filed an amended complaint to list New York, Washington, California, Illinois, Massachusetts, Ohio and Pennsylvania as plaintiffs, along with the federal government.
The merger of AT&T and T-Mobile would create the nation's largest mobile carrier, making Verizon Wireless second and leaving Sprint a distant third in the marketplace. If the deal were approved, Verizon Wireless and AT&T would account for more than 80 percent of the wireless market.
Sprint filed its own suit against the merger of its rivals earlier this month.
"We have had an excellent working relationship with a number of state attorneys general and they have provided invaluable assistance throughout our investigation," the Justice Department said in a statement. "We are pleased that these states have joined the department in its lawsuit. Together, we will seek to protect consumers from the anticompetitive harm that would result from this proposed transaction."
Michael Balmoris, an AT&T spokesman, said it is not unusual for state attorneys general to participate in Justice Department merger reviews and court filings.
"We appreciate that 11 state attorneys general and hundreds of other local, state and federal officials are publicly supportive of our merger," he said. "We will continue to seek an expedited hearing on the DOJ’s complaint. On a parallel path, we have been and remain interested in a solution that addresses the DOJ’s issues with the T-Mobile merger."
Public interest groups that oppose the merger praised the announcement.
"Their participation is an indication that states, too, recognize the tremendous harm that this deal would cause to consumers across the country and to our economy generally, and that they do not agree with AT&T's claims of benefits from the deal, including the creation of new jobs," said Gigi Sohn, president of Public Knowledge.
On Thursday, 15 House Democrats led by Rep. Heath Shuler (D-N.C.) wrote to President Obama urging his administration to swiftly settle the lawsuit. The lawmakers argued the deal would create thousands of jobs.
Updated at 3:56 p.m.