Seven states announced on Friday that they would join the Justice Department in challenging AT&T's proposed merger with T-Mobile.
Attorneys general from New York, Washington, California, Illinois, Massachusetts, Ohio and Pennsylvania will argue along with federal lawyers that combining the two major wireless providers would hurt competition and harm consumers.
The list of states was included in an amended complaint filed by the DOJ.
"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.
If a judge allows the $39 billion deal to go forward, and the Federal Communications Commission approves the transfer of spectrum licenses, the merger would create the largest wireless company in the U.S.
"Our review of the proposed merger between AT&T and T-Mobile has led me to conclude that it would hinder competition and reduce consumer choice," California AG Kamala Harris said in a statement.
Critics of the merger hailed the decision by the attorneys general to challenge the merger.
"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," Public Knowledge President Gigi Sohn said in a statement.
At least 11 state attorneys general voiced support for the merger before the Justice Department announced that it would go to court to block it, according to AT&T.
"It is not unusual for state attorneys general to participate in DOJ merger review proceedings or court filings," an AT&T spokesperson said in a statement. "At the same time, we appreciate that 11 state attorneys general and hundreds of other local, state and federal officials are publicly supportive of our merger."
AT&T says it will continue to press for an expedited court process and remains confident that the merger will go forward.