The proposed merger of Northwest and Delta Airlines is to be examined by the US Justice Department to gauge the likely effects on competition.
The department declined to indicate any sort of time frame for its investigation.
The two airlines announced details of their proposed tie-up on Monday. The merger, if it goes ahead, will create the largest airline in the world.
Under the terms of the merger deal, Northwest shareholders would receive 1.25 Delta shares for every Northwest share they own.
As well as possible anti-trust hurdles, the merger is likely to face opposition from Northwest Airlines employees who are likely to face job losses once the merger is effected.
The new airline will assume Delta’s name and be based in Atlanta, the current HQ of Delta Airlines. Northwest’s base in Minnesota will be retained as a hub of the merged airline’s network.