Both the lawsuit and the investigation center on whether American, Delta, Southwest, and United "conspired to restrict capacity by limiting routes and the number of available seats in order to charge artificially high prices," as a press release announcing the class action antitrust lawsuit stated. No evidence to support the claims Not surprisingly, the airlines don't believe that they have broken the law. American Airlines CEO Doug Parker recently sent employees a letter asserting that the airline has done nothing wrong, according to the Los Angeles Times. American Airlines CEO Doug Parker recently struck back at allegations of collusion. Image source: American Airlines Given the acceleration in U.S. airline capacity growth in 2015 and the corresponding decline in fares, Parker's denial appears to be justified. But in the Dallas-Fort-Worth area -- the subject of the lawsuit -- the idea that the airlines are colluding is even more preposterous.