American Airlines has sued a website that purports to “expos[e] loopholes in airfare pricing to save you money,” saying the site is engaging in deception. The airline is also threatening to cancel tickets purchased through the site.

The website, Skiplagged, centers on a controversial technique called skiplagging or hidden-city flying, which involves buying a ticket that includes a connecting flight, but getting off at the layover airport and skipping the second flight. For example, a traveler who wants to go to Charlotte may find it’s cheaper to buy a ticket to New York through Charlotte than buying a trip to Charlotte. To skiplag the trip, the flyer would buy the cheaper New York ticket but just walk out the Charlotte airport rather than getting on the New York flight.