A growing number of airlines, including Southwest, United and, now, American, say they have found suspect replacement parts in their engines.

AMR Corp’s American became the latest carrier to identify improperly certified parts from UK-based AOG Technics Ltd. on their planes, Bloomberg News reported Thursday.

AOG, which supplies parts to outside companies that do repair work on aircraft, allegedly sold thousands of engine parts to a number of carriers that had falsified paperwork, Aviation Week reported. AOG stands for “aircraft on the ground,” commonly used acronym in the industry to denote planes that cannot fly for maintenance or repair reasons.

On Sept. 20, London’s High Court ruled that the company had two weeks to send over details about the CFM56 and CF6 engines it has sold, Aviation Week added. The CFM56 engine, which is used on a variety of single-aisle Boeing 737 and Airbus A320 aircraft, is the top-selling model globally.