The Department of Justice is seeking additional details of AMR Corp AAMRQ.PK and US Airways Group Inc's LCC.N proposed $11 billion merger which aims to create the world's largest airline.
The companies said they expect to respond promptly to the "second request" from the Justice Department and they continue to expect the deal to be completed in the third quarter of 2013.
So-called "second requests" are quite common in anti-trust reviews of deals.
The Justice Department has rarely challenged an airline merger in recent years. The last one to be challenged was a proposed United-US Airways deal in 2000-2001.
However, to preserve competition, U.S. antitrust regulators typically require airlines to sell overlapping routes before approving a merger.
The DOJ did not return a call seeking comment.
The tie-up -- the fourth major merger in the U.S. airline industry since 2008 -- is expected to create the world's largest airline, with 6,700 daily flights, making the merged company two percent larger than current No. 1, United Continental Holdings (UAL.N) in terms of traffic.
(Reporting by Adithya Venkatesan Editing by Andre Grenon and Edwina Gibbs)