DALLAS (CBSDFW.COM) – Southwest Airline will pay $15 million dollars to settle class-action lawsuits.

The lawsuits accuse the four biggest U.S. airlines of conspiring to boost prices by holding down the supply of tickets for sale.

Dallas-based Southwest Airline denies breaking the law, but says it settled to avoid the expense of more litigation.

Officials with American Airline and United Airline say they will continue to defend themselves against the lawsuits. Representatives for Delta Airline did not immediately comment.

Lawyers for consumers filed dozens of lawsuits against the airlines in mid-2015.

The feds demanded documents from the airlines to see if they were colluding to limit capacity, and the number of seats to keep prices high.

The U.S. Justice Department hasn’t taken any further action.