Washington Post:

The attorney generals of California, Illinois, Massachusetts, New York, Ohio, Pennsylvania, and Washington joined the suit filed in the U.S. District Court of the District of Columbia. They argued that the deal would violate antitrust laws by reducing the number of wireless carriers from four to three national companies and removing an important lower-cost competitor.

By piling on the existing Department of Justice lawsuit, the states may make it less likely that Justice will accept a settlement instead of forcing the merger to stop altogether, according to a legal expert consulted for the article.