The US Justice Department has confirmed it has launched an investigation into ‘possible unlawful co-ordination by some airlines’.
It did not name which airlines were being investigated but it is believed the probe is centred on whether airlines have been conspiring to limit their combined growth to keep ticket prices high.
The Associated Press had earlier reported that it had seen a document revealing the Justice Department had requested information from airlines concerning their communication with each other.
The probe comes after years of mergers and restructuring by the major US carriers.
Delta bought out Northwest in 2008, Southwest purchased Airtran in 2011, United Airlines bought Continental in 2012 and US Airways merged with American Airlines in 2013.
News of the investigation hit US airline share prices and the Dow Jones airline index fell almost 4% in early trading.