Justice announced Tuesday that it was joining a lawsuit filed by six state attorneys general that argues the merger would violate federal antitrust laws.
The airlines, which are currently the nation’s fourth and fifth largest carriers, announced earlier this year that they would combine forces in an $11 billion deal.
Justice said the merger would violate federal antitrust laws because the companies offer too many similar flights. The agency argued that the combination of the two airlines into one would result in fewer options for passengers and potentially higher prices.
US Air and American have argued that their route structures are “complimentary” since they announced their intentions to merge, arguing that they currently have only 12 similar nonstop flights.
The DOJ and other opponents of the merger argue that if connecting flights are included, the overlap covers more than 1,000 flights.
US Air and American vowed on Tuesday to fight the Justice Department’s attempt to block their merger.
The companies have argued since February that it is necessary for them to merge to compete with other airlines that have been allowed to combine forces in recent years.
The proposed US Airways-American merger follows previous combinations by Delta and Northwest Airlines, United and Continental Airlines and Southwest and AirTran Airways, all since 2008.
The Justice Department did not intervene in any of those earlier mergers.