The four biggest U.S. airlines were sued by travelers claiming the carriers are conspiring to keep ticket prices high in the face of falling fuel prices.

The consumers’ case comes just after the U.S. Justice Department confirmed it’s investigating possible collusion.

Delta Air Lines Inc., United Continental Holdings Inc., American Airlines Group Inc. and Southwest Airlines Co. were sued July 2 in Chicago federal court. A day earlier, American Airlines and Southwest acknowledged receipt of Justice Department requests for information on passenger capacity. [Full text of lawsuits is embedded below.]

The Chicago lawsuit is at least the second such claim filed.

Those suits and the Justice Department probe come two years after the federal government sued to block American’s merger with US Airways. The merger went ahead after the case was settled. The DOJ said then that largest carriers “increasingly prefer tacit coordination over full-throated competition.”

Resolution of that case allowed American to emerge as the world’s biggest airline and ended an era of consolidation that also saw the disappearance of America West, Continental and AirTran as stand-alone airlines.

“This increased consolidation has hurt airline passengers,” according to the Chicago-filed complaint. “Defendants have, in tandem, raised fare, imposed new and higher fees on travelers and reduced their capacity and service,” leaving U.S. customers with fewer choices.

Time Covered

The suing fliers are seeking to represent anyone who bought a ticket for domestic air travel from Oct. 1, 2012, to now, together with unspecified money damages. A complaint making similar allegations was filed July 1 at the federal court in White Plains, New York.

American’s and United’s media relations departments did not immediately respond to telephone messages seeking comment on the litigation.

The Chicago case is Bidgoli v. American Airlines Group Inc., 15-cv-5903, U.S. District Court, Northern District of Illinois (Chicago). The New York case is Devivo v. Delta Airlines Inc., 15-cv-5162, U.S. District Court, Southern District of New York (White Plains).

This article was written by Andrew Harris from Bloomberg and was legally licensed through the NewsCred publisher network.

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Photo Credit: Passengers waiting for baggage at New York's Newark International Airport. Skift