A group of passengers is suing U.S. airlines over allegations they have colluded to keep airfares high since a string of mergers consolidated the industry. The lawsuits, filed in New York and Illinois, accuse Delta, American, United and Southwest airlines of engaging in a "conspiracy to fix, raise, maintain, or stabilize prices of airline tickets through a number of mechanisms." The lawsuit was filed after the Department of Justice announced its own antitrust investigation.