American Airlines and US Airways want to know why the federal government allowed previous airline mergers but seeks to block theirs.
The airlines say they will ask a judge to order the U.S. Department of Justice to turn over documents about four previous mergers between 2005 and 2011.
They made the request in a filing Friday with the U.S. district court in Washington. A trial is scheduled to begin Nov. 25 on the government's lawsuit against the merger, which would create the world's largest airline.
The Justice Department says the merger will limit competition and raise prices. The airlines maintain there will be plenty of competition.
The carriers want to know why regulators approved four deals -- Delta and Northwest, United and Continental, Southwest and AirTran and US Airways and America West -- but now argue that the American-US Airways merger would hurt consumers.
They argue that all the deals are similar and that the Justice Department has made an "abrupt and unexplained reversal" in how it views airline mergers.
The Justice Department did not immediately respond to a request for comment.
The airlines' lawyers said the Justice Department claims that documents about the previous mergers aren't relevant and that the government can keep such deliberations confidential.
They argued that the documents would help the airlines prove that the merger would boost competition when viewed through the same economic models and analyses that the government used to approve the earlier deals.
The Justice Department has argued that the previous mergers didn't produce the benefits that it expected, such as more competition and improved service, and that the American-US Airways combination would make things even worse.
American, which is owned by AMR Corp., and US Airways Group Inc. argue that together they would form a strong and roughly equal rival to United and Delta, the world's two biggest airlines.