A class-action lawsuit filed last month alleges that the four biggest U.S. airlines - American, Delta, Southwest and United - conspired to keep ticket prices high.

The lawsuit claims that the price-fixing between the four airlines - which control about 80 percent of the market for domestic air travel - began no later than July 2, 2011.

The lawsuit seeks class-action status for those who purchased domestic flights from the airlines within the last four years. The suit also seeks an unspecified amount in damages.

Heather Hansen explained on "Happening Now" today that this lawsuit is well-timed to coincide with the Justice Department launching its own investigation into the airlines.

Jonna Spilbor said the crux of this case is if the airlines are manipulating supply and demand to keep their prices up.

She added that the Justice Department has a tough road ahead to find anything damning, as there will likely be no emails or hard evidence of conspiracy.

Watch the "Happening Now" clip above.

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