Qatar Airways — a Middle East-based carrier with deep pockets and several United States gateways — has for several years been a thorn in the side of U.S.-based carriers such as United Airlines, Delta Air Lines and American Airlines, all of which worry that Qatar could siphon off passengers and jobs in the U.S. if left unchecked. The three U.S. carriers have been working with U.S. aviation officials to try to ensure Qatar is held in check.