DALLAS >> Southwest Airlines will pay $15 million to settle class-action lawsuits that accuse the four biggest U.S. carriers of conspiring to boost prices by holding down the supply of tickets for sale.

Southwest denies breaking the law but says it settled to avoid the expense of more litigation.

American and United said Thursday they would continue to defend themselves against the lawsuits. Delta did not immediately comment.

Lawyers for consumers filed dozens of lawsuits against the airlines in mid-2015 after the Associated Press first reported that the Justice Department was investigating possible collusion.

Federal officials demanded documents from the airlines to see if they were colluding to limit capacity — the number of seats — to keep prices high. The Justice Department hasn’t taken any further action.